Coronaviruses (CoVs) comprise a large family of enveloped, single-stranded, zoonotic RNA viruses belonging to the family
Coronaviridae, order Nidovirales ( Fig. 1). They can infect a variety of animals (including livestock, companion animals and birds), in which they can cause serious respiratory, enteric, cardiovascular and neurologic disease. 1 2 , In humans, CoVs mostly cause respiratory and gastrointestinal symptoms ranging from the common cold to more severe disease such as bronchitis, pneumonia, severe acute respiratory distress syndrome (ARDS), coagulopathy, multi-organ failure and death. 3 Human coronaviruses (HCoVs) have also been associated with exacerbations of chronic obstructive pulmonary disease, 4–8 cystic fibrosis 9 and asthma. 10 11 , 12 FIGURE 1.:
Summary of coronavirus diseases. COVID-19 indicates coronavirus disease 2019.
CoVs are classified into
Alphacoronaviruses and Betacoronaviruses (which are mainly found in mammals such as bats, rodents, civets and humans) and Gammacoronaviruses and Deltacoronaviruses (which are mainly found in birds). 8 , 13 , Four CoVs commonly circulate among humans: HCoV2-229E, -HKU1, -NL63 and -OC43. 14 15 , These viruses are believed to have originally derived from bats (NL63, 229E), 16 17 , dromedary camels (229E) 18 and cattle (OC43). 19 The origin of HCoV-HKU1 remains unknown. Several CoVs are known to circulate in animals (with bats acting as the main reservoir) but have not been associated with human infection. 20 3 , 21 , CoVs are capable of rapid mutation and recombination leading to novel CoVs that can spread from animals to humans. This occurred in China in 2002 when the novel CoV severe acute respiratory syndrome coronavirus (SARS-CoV) emerged, thought to have been transmitted from civet cats or bats to humans. 22 Another novel CoVs emerged in Saudi Arabia in 2012, Middle East respiratory syndrome coronavirus (MERS-CoV), which is transmitted from dromedary camels to humans. 22–25 26 , The 2019 novel CoV (SARS-CoV-2), which originated in China and is currently causing outbreaks globally, is a novel 27 Betacoronavirus belonging to the lineage B or subgenus sarbecovirus, which includes SARS-CoV. Sequencing shows that the genome is most closely related (87%–89% nucleotide identity) to the bat SARS-related CoV found in Chinese horseshoe bats (bat-SL-CoVZC45). 28 28 , The outbreak of SARS-CoV-2 started in Wuhan city, Hubei province, China, where The Health Commission of Hubei province first announced a cluster of adults with pneumonia of unexplained etiology on December 31, 2019. A local seafood and animal market was identified as a potential source. However, the main driver of the outbreak is symptomatic and asymptomatic humans infected with SARS-CoV-2 from whom the virus can spread to others through respiratory droplets or direct contact. 29 From Wuhan city SARS-CoV-2 has spread to other Chinese cities and internationally, threating to cause a global pandemic. The term COVID-19 is used for the clinical disease caused by SARS-CoV-2. 28 30
In this review, we summarize epidemiologic, clinical and diagnostic findings, as well as treatment and prevention options for common circulating and novel CoVs infections in humans with a focus on infections in children.
Common Circulating HCoVs
Common circulating HCoVs can be isolated from 4% to 6% of children hospitalized for acute respiratory tract infections
11 , 15 , and from 8% of children in an ambulatory setting ( 31 Table 1). 15 , 32 , Children under the age of 3 years and children with heart disease are the most frequently affected. 33 4 , 15 , 35 , Reinfections later in life are common 36 32 , 115 , despite the fact that most individuals seroconvert to HCoVs during childhood. 116 In contrast to other respiratory tract viruses [eg, respiratory syncytial virus (RSV)], there is no decrease in the relative prevalence of HCoVs infections with increasing age. 117–120 4 , 5 , 15 , 36 TABLE 1.:
Characteristics of Human Coronaviruses
In 11%–46% of cases, common circulating HCoVs are found as coinfections with other respiratory viruses such as adeno-, boca-, rhino-, RSV, influenza or parainfluenza virus.
5 , 15 , 16 , 31–33 , 36 , 79 , 81 , 121 , Symptomatic children whose only detectable respiratory virus is a HCoV are reported to more likely suffer from an underlying chronic disease compared with children coinfected with other respiratory viruses. 122 31
Of the 4 common circulating HCoVs, NL63 and OC43 are the most frequently isolated species.
4 , 11 , 15 , 35 , Cyclical patterns have been observed for 229E and OC43, with outbreaks occurring every 2–4 years. 36 4 , 15 , 32 , 35 , 82 , 116 , Seasonal patterns have also been observed: in the Northern Hemisphere, common circulating HCoVs mostly cause infections in humans between December and May, and in the Southern Hemisphere between March and November with peaks in late winter/early spring for 229E and OC43 and in autumn for NL63. 119 4 , 5 , 11 , 15 , 32 , HCoV-HKU1 has been reported to mainly occur in spring and summer in Hong Kong, 123 11 , but in winter and spring in the United Kingdom and Brazil. 124 4 , 15 SARS-CoV and MERS-CoV
SARS-CoV is a novel group 2b
Betacoronavirus which initially emerged in Guangdong province, south China in 2002, then spread to Hong Kong and from there rapidly to many other countries. 23–25 It caused severe lower respiratory tract infection with a severe morbidity and a high case-fatality rate (approaching 50% in individuals over 60 years of age, overall 10%). 125 63 , 106 , 107 , Person-to-person transmission of SARS-CoV is well established. 126 The virus has spread to 29 countries and has been estimated to have caused more than 8000 infections and 774 deaths worldwide ( 55 Table 1). 52
MERS-CoV is a novel group 2c
Betacoronavirus which first appeared in Saudi Arabia in 2012. 26 , 27 , It can spread from person-to-person 127 and can cause severe lower respiratory tract infections with a case-fatality rate of 20% to 40%. 128 67 , 106 , Apart from being endemic in the Middle East, there was a nosocomial outbreak of MERS-CoV in South Korea in 2014, involving 16 hospitals and 186 patients, caused by a medical doctor returning from the Middle East. 108–112 49 , MERS-CoV spread to 27 countries causing an estimated 2494 infections and 858 deaths ( 68 Table 1). 53
The overall reproductive number (R0) for SARS-CoV was estimated to be 0.3–2.9
37 , 39 , 40 , 42 , 43 , and for MERS-CoV to be 0.5–3.5 ( 47 Table 1). 39 , 46 , R0s largely depend on geographic location, stage of the outbreak and inclusion of only nosocomial versus general transmission. Both viruses have been associated with early super-spreading events with R0s of up to 22 for SARS-CoV 48 39 , 40 , and up to 30 for MERS-CoV. 43 39 , These large numbers of secondary infections have been mostly associated with nosocomial outbreaks: 30% of all SARS-CoV cases (mostly health care workers) and 44%–100% of all MERS-CoV cases (mostly patients) occurred from nosocomial transmissions. 49 39 , 55 , These super-spreading events were followed by reduced spread in the following generations of viruses with a decrease in the R0s to 0.8 for SARS-CoV 56 and to 0.7 for MERS-CoV ( 39 Table 1). Therefore, both SARS-CoV and MERS-CoV have low potential for long-term sustained community transmission. No human SARS-CoV infections have been detected since July 2003. However, SARS-CoV-like viruses can be found in bats, which are known to be able infect human cells without adaptation, making it possible for SARS-CoVs to reemerge 128 (as has now happened with SARS-CoV-2). The zoonotic transmission of MERS-CoV to humans has continued, attributed to the role of dromedary camels as a reservoir and their close contact with humans (in contrast to human-bat-interactions). 84 21 SARS-CoV-2
Early in the SARS-CoV-2 outbreak, it was shown that person-to-person transmission was the main driver.
The R0 for SARS-CoV-2 is currently estimated at 2.7. 28 The incubation period is estimated at 5–6 days, which is similar to that for SARS-CoV and MERS-CoV. 38 38 , 63–65 , The serial interval is estimated to be 8 days, also similar to the other novel CoVs ( 67–72 Table 1). 38 , 45 , 48 , By March 2020, the World Health Organization reported that SARS-CoV-2 had spread to over 100 countries and caused over 100,000 infections and over 3500 deaths. 70 At that time the case-fatality rate was uncertain but estimated at 0.9%–3%, 54 54 , 113 , which is much lower than for SARS-CoV and MERS-CoV (6%–17% and 20%–40%, respectively). 114 63 , 67 , 106–112 SYMPTOMS
In children, common circulating HCoVs can cause common cold symptoms such as fever,
5 , 11 , rhinitis, 32 5 , otitis, 11 pharyngitis, 5 5 , laryngitis 11 and headache, 5 5 , 16 , but also bronchitis, 81 5 , bronchiolitis, 11 5 , wheezing, 11 4 , 11 , pneumonia, 32 5 , 81 , and, in up to 57% of cases, gastrointestinal symptoms (which are more common in children than adults). 82 In a study including children and adults, fatigue, headache, myalgia and sore throat were more common in HCoV-infected patients compared with RSV-infected patients, while fever, cough and dyspnea were more frequent in the later. 5–7 Fewer patients infected with HCoVs had fever compared with those infected with RSV or influenza. 36 36
In children, HCoV-NL63 has been associated with conjunctivitis,
croup, 78 11 , 79 , asthma exacerbations, 80 11 , febrile seizures 12 and HCoV-HKU1 with febrile seizures. 11 Rare cases of neurologic diseases have also been described (eg, the detection of HCoV in cerebrospinal fluid in a child presenting with acute disseminated encephalomyelitis 7 or in cerebrospinal fluid of adults with multiple sclerosis.) 83 129 , A suspected association between HCoVs and Kawasaki disease could not be confirmed. 130 131 , Common HCoVs can be isolated from asymptomatic individuals. 132 During an infection, the viral load is high in the first 2 days and decreases thereafter. 16 A correlation between viral load and severity of disease has not been observed 29 This contrasts with SARS-CoV for which a higher initial viral load is independently associated with a worse prognosis, including a higher case-fatality rate. 29 133 , Virus particles can be isolated from nasopharyngeal secretions up to 14 days after the onset of infection. 134 135 SARS-CoV
There are 3 case series that report a total of 41 children who were affected by SARS-CoV.
The virus was associated with milder disease in children compared with adults, and no deaths have been reported in children. 57–59 57–59 , Symptomatic children with SARS-CoV infection were reported to have fever (91%–100%), 86 myalgia (10%–40%), 57–59 57 , rhinitis (33%–60%), 58 sore throat (5%–30%), 57–59 cough (43%–80%), 57–59 dyspnea (10%–14%), 57–59 38 , headache (14%–40%) 84 and, less commonly, vomiting (20%), 57–59 57 , abdominal pain (10%), 59 diarrhea (10%) 57 58 , and febrile seizures (10%). 59 In total, 50%–80% of children had other family members who were infected 57 and 30% had a nosocomial contact with SARS-CoV. 57–59 Most children recover quickly from an infection with SARS-CoV. 57 However, abnormalities on chest computed tomography (CT) can persist for several months (eg, air trapping and ground-glass opacifications). 86 136
There is no evidence that SARS-CoV can be vertically transmitted to the fetus.
However, SARS-CoV infections during pregnancy have been associated with possible miscarriage, intrauterine growth retardation and preterm delivery. 137 137 , 138 MERS-CoV
Most case series of patients infected with MERS-CoV report a low proportion (0.1%–4%) of children.
34 , 76 , 109 , 110 , 139 , In a large case series of 2235 children with acute respiratory tract infection who presented to a tertiary hospital in Saudi Arabia during the MERS-CoV epidemic (2012–2013), none tested positive for MERS-CoV ( 140 Table 1). There are 2 small case series of children infected with MERS-CoV: one including 31 children with a mean age of 10 years 34 and the other one only 7 children. 60 In both studies, 42% of children were asymptomatic. 76 60 , In the case series of 7 children, 57% suffered from fever, 28% from vomiting and diarrhea and 14% from cough and shortness of breath. 76 Two children required oxygen supplementation and one mechanical ventilation. 76 In the other case series, 2 died (6%). 76 The main sources of MERS-CoV infection in children were household (32%) and other contacts (23%), followed by nosocomial transmission (19%). 60 60
Eight cases of MERS-CoV maternal infections during pregnancy have been reported (occurring between 20 and 28 weeks of pregnancy), three of the affected infants died.
Different case definitions for COVID-19 cases in adults and children from authoritive sources as of March 2020 are detailed in
Table 2. Children are less commonly affected by SARS-CoV-2, the Chinese Centers for Disease Control and Prevention reports that of the 72,314 cases reported as of February 11, 2020, only 2% were in individuals of less than 19 years of age. There are 3 case series of children who have been infected with SARS-CoV-2. 114 61 , 72 , The first included 20 children up to January 31, 2020, in the Province of Zhejiang, 77 the second 34 children between January 19, 2020, and February 7, 2020, in the Province of Shenzhen, 72 and the third 9 infants from different provinces in China. 61 The case series with 34 children provides the most clinical details: none of the children had an underlying disease, 65% had common respiratory symptoms, 26% had mild disease and 9% were asymptomatic. 77 The most common symptoms were fever (50%) and cough (38%). 61 In the case series of 20 children, presentation was with low to moderate or no fever, rhinitis, cough, fatigue, headache, diarrhea and, in more severe cases, with dyspnea, cyanosis and poor feeding, but the numbers were not specified. 61 In the series of 9 infants, only 4 were reported to have fever. One infant was asymptomatic. 72 Additional asymptomatic children infected with SARS-CoV-2 outside these case series have also been described (eg, a 10-year-old asymptomatic child with radiologic ground-glass lung opacities on chest CT). 77 Most infected children recover 1–2 weeks after the onset of symptoms and no deaths from SARS-CoV-2 had been reported by February 2020. 28 72 TABLE 2.:
Case Definitions for SARS-CoV-2 Infections in Adults and Children (as of February 2020)
From these series, it appears that children have milder clinical symptoms than adults
61 , (as has been reported for SARS-CoV and MERS-CoV infections), 72 57–60 , 76 , which could mean children might not be tested for SARS-CoV-2 as frequently as adults. It has therefore been suggested that asymptomatic or mildly symptomatic children might transmit the disease. 86 However, the majority of children infected with SARS-CoV-2 thus far have been part of a family cluster outbreak [100% in the infants series, in which other family member had symptoms before the infants in all cases; 82% in the case series of 34 children; 147 and the majority in the one with 20 children (exact number not specified)]. 61 This is similar to SARS-CoV, in which 50%–80% 72 of children were reported to have an affected household contact 57–59 and to MERS-CoV in which it was 32%. 60 60
A study prepublished in early March 2020 suggests that children are just as likely as adults to become infected with SARS-CoV-2 but are less likely to be symptomatic or develop severe symptoms.
However, the importance of children in transmitting the virus remains uncertain. 246
From a small case series of 9 mothers who were infected with SARS-CoV-2, there is, to date, no evidence that SARS-CoV-2 can be vertically transmitted to the infant.
148 LABORATORY FINDINGS
Laboratory findings from children are similar with infections caused by different novel CoVs (
Table 1). The white blood cell count is typically normal or reduced with decreased neutrophil and/or lymphocyte counts. 85 57–59 , 72 , Thrombocytopenia may occur. 86 57–59 , 76 , C-reactive protein and procalcitonin levels are often normal. 86 In severe cases, elevated liver enzymes, 72 57–59 , 72 , lactate dehydrogenase levels, 86 as well as a abnormal coagulation and elevated 57 d-dimers have been reported. 57–59 , 72 , 86 SARS-CoV-2
The same laboratory findings has above have been observed for children infected with SARS-CoV-2.
In the case series of 34 children, the white blood cell count was normal in 83%, neutropenia and lymphopenia were each found in 1 case (3%). The lactate dehydrogenase level was elevated in 30% of cases. 61 C-reactive protein and procalcitonin levels were each elevated in 1 case only (3%). 61 RADIOLOGIC FINDINGS
Similar to the laboratory findings, radiologic findings from children are also similar across infections with different novel CoVs (
Table 1). On chest radiography, children mostly show bilateral patchy airspace consolidations often at the periphery of the lungs, peribronchial thickening and ground-glass opacities. 57–59 , 76 , 86 , Chest CT mostly shows airspace consolidations and ground-glass opacities. 87 89 SARS-CoV-2
CT changes observed in children infected with SARS-CoV-2 include bilateral multiple patchy, nodular ground-glass opacities, speckled ground-glass opacities and/or infiltrating shadows in the middle and outer zone of the lung or under the pleura.
61 , These findings are unspecific and milder compared with those in adults. 88 88 DIAGNOSIS
The main basis for diagnosis of infections with HCoVs is real-time polymerase chain reaction (RT-PCR) on upper or lower respiratory secretions.
5 , 15 , For SARS-CoV, MERS-CoV and SARS-CoV-2, higher viral loads have been detected in samples from the lower respiratory tract compared with the upper respiratory tract. 90–96 28 , Therefore, in clinically suspected cases with an initially negative result on nasopharyngeal or throat swab, repeat testing of upper respiratory tract samples or (preferably) testing of lower respiratory tract samples should be done. RT-PCRs on stool samples can be positive for HCoVs but is not used for routine diagnosis. 149 91 , 98 , For SARS-CoV and SARS-CoV-2, rare cases with positive PCRs in blood have been reported. 99 28 , Serology has been used to diagnose infections with SARS-CoV and MERS-CoV, but is not useful in the acute phase of the infection. 150 Cross-reactivities between antibodies against SARS-CoV and common CoVs have been observed. 100–103 151 SARS-CoV-2
Whole genome sequencing allowed the rapid development of molecular diagnostic tests for SARS-CoV-2.
RT-PCR for genes encoding the internal RNA-dependent RNA polymerase and surface spike glycoprotein are commonly used. 28 28 TREATMENT
Supportive treatment including sufficient fluid and calorie intake, and additional oxygen supplementation should be used in the treatment of children infected with HCoVs. The aim is to prevent ARDS, organ failure and secondary nosocomial infections. If bacterial infection is suspected broad-spectrum antibiotics such as second or third generation cephalosporins may be used.
In the absence of specific antiviral drugs for CoVs, broad-spectrum antiviral drugs, such as interferon alpha and beta or ribavirin were used for the treatment of SARS-CoV, including in children.
Ribavirin was subsequently shown to be ineffective or even harmful because it can cause hemolytic anemia or liver dysfunction. 57–59 In adults, interferon-alpha alone or together with ribavirin also did not consistently improve outcomes. 152 152 , There is some evidence that intravenous corticosteroids led to clinical and radiologic improvement in SARS-CoV-infected individuals. 153 However, a systematic review showed that the evidence for this is inconclusive and corticosteroids might also be harmful (delayed viral clearance, avascular necrosis, osteoporosis, new onset of diabetes). 58 There is some evidence from adult studies that lopinavir/ritonavir (Kaletra) started early during infection is associated with improved clinical outcomes (decreased intubation, ARDS and death rates). 152 154 , However, a systematic review found inconclusive results for the use of lopinavir/ritonavir because of a possible selection bias in many of the studies. 155 Inconclusive results were also found for intravenous immunoglobulins because studies did not account for comorbidities, stage of illness and effect of other treatments. 152 There is no evidence for the use of monoclonal antibodies against tumor necrosis factor alpha. 152 156 MERS-CoV
There are no studies on treatment outcomes for MERS-CoV in children. In adults, as for SARS-CoV, interferon or ribavirin alone or in combination have not been shown to have a clear benefit.
Mycophenolate mofetil, which inhibits guanine (and therefore RNA) synthesis, was identified as a potential anti-MERS-CoV drug in vitro. 157–159 However, animal studies showed that the drug leads to worse outcomes with higher viral loads in lung and extrapulmonary tissues. 160 Consistent with this, renal transplant patients on mycophenolate mofetil have been reported to develop severe and sometimes fatal MERS-CoV infections. 161 162 SARS-CoV-2
Until the results of on-going clinical trials become available, there is no definitive evidence on which to base treatment of patients infected with SARS-CoV-2. The only treatment recommendation for children, published by the Zhejiang University School of Medicine, suggests the use of nebulized interferon alpha-2b and oral lopinavir/ritonavir together with corticosteroids for complications (ARDS, encephalitis, hemophagocytic syndrome or septic shock) and intravenous immunoglobulin for severe cases.
However, as none of these therapies have shown a clear benefit in the treatment of other novel CoVs, it is questionable whether they will be beneficial in the treatment of SARS-CoV-2. Neither the World Health Organization nor the US Centers for Disease Control and Prevention recommends any specific treatment in children or adults.
97 , Despite this, in the previously mentioned case series of the 34 children infected with SARS-CoV-2, 59% were treated with lopinavir/ritonavir. 163 None of the children received glucocorticoids or immunoglobulins. 61 61 Other Therapeutic Options
Despite their diversity, CoVs share many proteins among different species, which is helpful for the design of new drugs. One of them is the surface structural spike glycoprotein S, which is responsible for virus-cell interaction.
Monoclonal antibodies (from convalescent human plasma, animal plasma or manufactured) against the spike glycoprotein S have been shown to inhibit fusion of CoVs with human cells and to decrease mortality rate in SARS-CoV-infected patients. 164 A protein, which also inhibits the spike glycoprotein S, although it is not a monoclonal antibody, has been isolated from a red alga called Griffithsia. 165–171 However, to date, it has only been tested in animal studies. 172 172
Angiotensin-converting enzyme 2, dipeptidyl peptidase 4, aminopeptidase N, O-acetylated sialic acid are further host receptors for HCoVs and monoclonal antibodies against these proteins might be useful in treatment of infections.
However, rapid mutation of CoVs poses a potential problem, which might be diminished by using several monoclonal antibodies targeting different epitopes. 173–176 166 Protease Inhibitors
Endosomal and nonendosomal virus entry into cells can be reduced by inhibiting responsible proteases.
Papain-like proteases (PLpro) are involved in viral replication in CoVs and are further potential targets for treatment. Numerous PLpro inhibitors have been identified. However, none of them has been validated in in vivo studies. 177–179 180 , Moreover, PLpro enzymes differ between CoVs species, making PLpro inhibitors narrow-spectrum antiviral drugs against CoVs. 181 182
A further protein involved in viral replication is CoV main proteinase, which is inhibited by lopinavir. However, as previously mentioned, lopinavir (plus ritonavir) has been shown to be effective against CoVs in animal and nonrandomized studies of SARS-CoV-infected humans.
154 , However, as previously mentioned, these results are considered inconclusive because of potential selection bias. 161 152 Chloroquine
Chloroquine, which is commonly used against malaria and autoimmune diseases, increases the endosomal pH thereby inhibiting virus-cell fusion, and is therefore a potential broad-spectrum antiviral drug.
It also interferes with glycosylation of cellular receptors of SARS-CoV. 183 In addition, in vitro studies show that chloroquine inhibits entry and postentry stages of SARS-CoV-2 into cells. 184 Moreover, chloroquine possesses immune-modulating activity, which might enhance its antiviral effect in vivo. 185 185 RNA Synthesis Inhibitors
As previously mentioned, ribavirin, a guanosine analog has been shown to be ineffective or even harmful against SARS-CoV
and MERS-CoV. 152 Immucillin-A, a new adenosine analog that has recently been developed, inhibits the viral RNA polymerase of a wide range of RNA viruses, including SARS-CoV and MERS-CoV, 157–159 and might be useful in the treatment of other HCoVs. Furthermore, inhibitors of helicase (which are proteins unwinding double-stranded RNA into single strands during replication) might be useful in treatment of CoVs. 186 RNA synthesis inhibitors, which reduce the formation of double-membrane vesicles, a hallmark of CoV2 replication, have been identified as potential antiviral drugs. 187 188 , A double-stranded RNA activated caspase oligomerizer (DRACO) that targets long viral double-stranded RNA and induces apoptosis of infected cells, but spares healthy cells, might also be useful in the treatment of CoVs. 189 190 VACCINES
Several vaccines against HCoVs are in development with the aim of preventing infection, reducing disease severity and viral shedding. The main antigens for vaccine development are the structural spike glycoprotein S or its receptor-binding domain (RBD).
However, the propensity of CoVs to rapidly mutate and recombine poses a potential problem for vaccine development. 191 Furthermore, the enhanced disease after viral challenges postvaccination has been observed in animal models after several different vaccines. 192–194 195–197 Live-attenuated Vaccines
The advantage of live-attenuated vaccines is that they usually induce a robust and long-lasting immune response, including cellular and humoral immunity to many different antigens. In SARS-CoV animal studies, attenuated mutants with deletion of the structural E gene have been shown to induce neutralizing antibodies, reduce viral loads and protect from clinical symptoms of SARS-CoV infection.
In contrast, deletion of open reading frames had little or no effect on viral loads in vitro and in vivo. 198–200 Other strategies under development for live-attenuated vaccines against CoVs are genome rearrangement or gene knockouts. 201 These have the advantage that the vaccine virus cannot recombine with wild viruses. 202–204 Inactivated Vaccines
In mouse models, inactivated vaccines successfully induce cellular and humoral immunity (with many different neutralization antibodies) against SARS-CoV
191 , and humoral immunity against MERS-CoV. 205–207 208 , In a human phase 1 trial, inactivated vaccines against SARS-CoV were well tolerated and elicited neutralizing antibodies. 209 However, no challenge studies have been done in humans, and in monkey challenge studies, no clear evidence of protection was shown despite the induction of strong cellular and humoral responses. 210 Moreover, concerns have been raised that inactivated vaccines against SARS-CoV and MERS-CoV may lead to harmful immune and/or inflammatory responses postchallenge. 211 195 , 209 Subunit and Recombinant Vaccines
Subunit vaccines are purified antigens, usually combined with adjuvants and are the most popular method in the development of vaccines against novel CoVs. For SARS-CoV and MERS-CoV, these are mostly developed from spike glycoprotein S, RBD or nucleocapsid protein.
Some studies show that subunit vaccines given intranasally might induce stronger immune responses and mucosal immunity. 212–216 Several subunit vaccines have shown to be successful in animal challenging studies. 217 218–220
In a study in monkeys, recombinant RBD protein was used to successfully reduce viral loads in lungs and oropharynx and to prevent MERS-CoV pneumonia.
In mice, similar results were achieved using recombinant RBD protein vaccines from SARS-CoV. 218 221 Viral Vectors Vaccines
Adenovirus-based vectors encoding SARS proteins (eg, nucleocapsid protein, spike glycoprotein S and other membrane proteins) have been shown to be immunogenic in mice and rhesus macaques in whom they induced humoral and cellular vaccine responses.
222 , Adenovirus-based vaccines carrying parts MERS-CoV have been shown to reduce morbidity and mortality (undetectable or reduced pulmonary viral loads) in mouse models. 223 196 , Initially, pulmonary hemorrhages were observed postviral challenge. 224 However, adding a CD40 ligand to the vaccine enhanced immunogenicity and efficacy, and also prevented inadvertent pulmonary pathology, which makes this vaccine a promising strategy. 196 Nonetheless, preexisting immunity against adenovirus might reduce efficacy. This might be addressed by giving a viral-based vaccine followed by a recombinant vaccine as a booster. 196 A adenovirus-based MERS-CoV vaccine has moved into a phase I clinical trial. 225 226
One study, comparing an inactivated SARS-CoV vaccine with an adenovirus-based vaccine against SARS-CoV, found that the first led to higher humoral responses.
Adenovirus-based vaccines administered intranasally led to immunoglobulin A antibody production which has been associated with superior protection from virus replication in lungs. 227 This indicates that measuring serum neutralizing antibodies might not be a sufficient way of assessing vaccine efficacy for HCoV as mucosal immunity might be more important. 227
For SARS-CoV, a poxvirus has also been used as a vector for an intranasally and intramuscularly administered vaccine. This vaccine-induced neutralizing antibodies and reduced viral loads in the respiratory tract of challenged mice.
However, a similar vaccine used in ferrets led to increased liver damage after SARS-CoV challenge. 228 197
Further vector vaccines for SARS-CoV that have been tested in animals are based on recombinant parainfluenza virus,
229 , live-attenuated recombinant measles virus, 230 attenuated rabies virus 231 and attenuated 232 Salmonella. 233 DNA Vaccines
Vaccines containing DNA encoding the spike glycoprotein seem to induce a more robust response of neutralizing antibodies against MERS-CoV than vaccines only containing the RBD protein. They have been shown to protect rhesus macaques from MERS-CoV pneumonia.
234 , Three DNA vaccines against MERS-CoV have advanced into clinical trials. 235 236–238 OTHER STRATEGIES FOR CONTROLLING EMERGING CORONAVIRUSES
After quickly spreading across the globe, SARS-CoV was contained in 2003 after a highly effective global public health response. This highlights the urgent need for rapid and effectful strategies of infection control. One of the main challenges with novel CoVs is the high potential for nosocomial transmission.
Health care settings seem to increase the risk of viral transmission due to aerosol-generating procedures such as intubation and bronchoscopy. Appropriate hospital hygiene practices are therefore crucial to limit nosocomial outbreaks. The main aims are to effectively triage patients with fever, respiratory symptoms and a contact history 239 and to apply stringent infection control measures such as isolating patients and quarantine contacts as early as possible. Ideally, each patient is placed in a single negative pressure room. If this is not possible, patients and health care workers should be cohorted. 240 Protective gear should include water-resistant gowns, disposable gloves, N95 masks and goggles or face shields. 241 Only suction catheters and mechanical respirators with a closed-circuit system and viral filters should be used. 240 In contrasts, nebulizers, oxygen masks or nasal continuous positive airway pressure systems should not be used on an open ward. 240 240 , Needless to say, strict hand hygiene needs to be applied and visitors should be avoided or limited to an absolute minimum. HCoVs have been shown to persist on dry surfaces for up to 9 days. 241 The persistence depends on temperature (shorter duration at 30–40°C) and humidity (longer at higher humidity). 242–244 HCoVs, including novel CoVs, can be inactivated by heating to 56°C for 30 minutes or by using lipid solvents such as ethanol (>75%), isopropanol (>70%), formaldehyde (>0.7%), povidone-iodine (>0.23%), sodium hypochlorite (>0.21%), hydrogen peroxide (>0.5%), but not chlorhexidine. 245 72 , 244 SUMMARY
SARS-CoV, MERS-CoV and SARS-CoV-2 infections seem to affect children less commonly and less severely as compared with adults. This might be because children are less frequently exposed to the main sources of transmission (which until now has been disproportionally nosocomial) or because they are less exposed to animals. However, it could also be that children are less frequently symptomatic or have less severe symptoms and are therefore less often tested, leading to an underestimate of the true numbers infected. In relation to SARS-CoV-2, a study prepublished in early March 2020 suggests that children are just as likely as adults to become infected with this virus but are less likely to be symptomatic or develop severe symptoms.
However, the importance of children in transmitting the virus remains uncertain. The majority of children infected by a novel CoVs reported thus far have a documented household contact, often showing symptoms before them, suggesting the possibility that children are not an important reservoir for novel CoVs. The clinical, laboratory and radiologic features in children are similar for all novel CoVs, except more children infected with SARS-CoV presented with fever compared with SARS-CoV-2 or MERS-CoV. To date, no deaths in children have been reported for SARS-CoV or SARS-CoV-2, except (in the case of the former) for infants of mothers who were infected during pregnancy. 246 REFERENCES
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