Influenza is commonly seen as a serious illness only in the elderly and persons with chronic conditions that place them at an increased risk of complications. 1–3 Consequently vaccination is recommended only for adults older than 64 years of age and for children with high risk medical conditions. 4, 5 However, recent studies 6–8 have shown that influenza is clinically important also in otherwise healthy children (especially those younger than 2 years of age) because it can lead to substantial numbers of excess hospitalizations, medical visits and antibiotic use. On the basis of these data, many health authorities now encourage influenza vaccination of healthy children younger than 2 years of age who were previously excluded from vaccination programs. 9
Vaccinating healthy children older than 2 years of age has not been recommended because of the apparently benign natural history of the disease. However, several studies 10–14 have shown that influenza is very common among healthy children attending day care, nurseries and schools, and it can have a significant impact on these children and their families by affecting the quality of life and disrupting usual activities. Children also shed greater quantities of influenza viruses than adults for a longer period of time, and they are an important source of infection to families and the community. 9, 15, 16
The conclusions from many previous studies of influenza are limited because of lack of laboratory confirmation of influenza in the children. Therefore it is difficult to determine whether the socioeconomic impact of influenza justifies vaccinating all children regardless of their age or underlying chronic disorders.
We conducted a prospective study in Italy during the 2001 to 2002 influenza season. This study comprised a total of 3771 children younger than 14 years of age (1990 boys; mean age ± sd, 3.37 ± 2.86 years) with symptoms of respiratory tract infection seen in emergency departments (2970 children) or by primary care pediatricians (801 children). Influenza viruses in the children were identified by culture and/or polymerase chain reaction. In addition to the participating children, we collected data from the household contacts of the children to assess the impact of a child’s influenza on respiratory morbidity and absenteeism from work or school in the parents and siblings and to determine the need for help at home to care for the sick children.
Before the onset of the 2001 to 2002 influenza season in Italy, we also randomized 303 healthy children aged 6 months to 5 years (163 boys; median age, 3.2 years) to receive either intramuscular virosomal influenza vaccine (Inflexal V; Berna Biotech, Berne, Switzerland; n = 202) or no vaccination (n = 101). During the influenza season information regarding respiratory illnesses and related morbidity among the study subjects and their household contacts was obtained by means of biweekly telephone interviews and monthly medical visits by trained investigators using standardized questionnaires.
Impact of influenza on children
Influenza viruses were identified in 352 (9.3%) of 3771 children with respiratory infection: 260 (8.7%) of 2970 children seen in the emergency departments; and 92 (11.5%) of 801 children treated by the primary care pediatricians (P = 0.022). We found influenza A viruses in 183 (52.0%) cases [129 (70.5%) of subtype H3N2 and 54 (29.5%) of subtype H1N1] and influenza B viruses in 169 (48.0%) cases. When we compared influenza-positive children with those with other respiratory infections, we found that although there were similar numbers and lengths of hospital stay and a similar number of additional medical visits, children with influenza had longer durations of fever and absenteeism from day care or school (Table 1). There were no significant differences in the social impact of influenza between children seen in emergency departments or by primary care pediatricians, between those with influenza A or B infections or between different age groups of influenza-positive children.
In the vaccine study children who had received the influenza vaccine had significantly fewer respiratory tract infections, antibiotic and antipyretic prescriptions and missed school days than did the unvaccinated control children (Table 2). 17
Impact of influenza on household contacts
Among the 915 household contacts of the 352 children with confirmed influenza, there were significantly more medical visits and missed work or school days and a greater need for help at home to care for the ill children than among the 9128 household contacts of the 3419 children without influenza (Table 3). The need for extra help at home was significantly longer for children with influenza B than with influenza A infections (1.30 ± 2.35 vs. 0.77 ± 1.82 days, P = 0.0016) and for influenza-positive children 2 to 5 years of age and >5 years than for those <2 years of age (1.01 ± 1.99 and 1.60 ± 2.44 vs. 0.71 ± 2.01 days, respectively; P = 0.004 and P = 0.011).
Among the household contacts of children in the vaccine study, the family members of influenza-vaccinated children had significantly fewer respiratory tract infections, medical visits and missed working days, and they required significantly less help at home to care for the sick children than the family members of unvaccinated children (Table 4). 17
The main findings of our study are those indicating the substantial clinical and socioeconomic impact of influenza on healthy children and their household contacts and the positive effect of influenza vaccination in reducing the related morbidity. These data support a wider pediatric use of influenza vaccine even in the absence of high risk conditions.
The considerable influenza-related medical and socioeconomic burden even in otherwise healthy children and their families demonstrated in this study confirm previous surveys that have shown that influenza has multiple effects, including significantly more school and parental work absenteeism and secondary illnesses in family members. 6–8, 14 Recently Neuzil et al. 14 estimated that there were 28 more episodes of illness and 63 missed school days for every 100 children followed up during the 2000 to 2001 influenza season in the Seattle area. 14
The increase in influenza illnesses among household contacts of children with influenza causes a number of socioeconomic problems. Children were first shown as the major transmitters of influenza during the 1957 pandemic. A study of a large general practice population outside London, UK, showed that adults who lived with schoolchildren were 2 to 3 times more likely to catch influenza than those who did not. 18 Other studies of influenza spread in the US in the 1970s confirmed that children were the main introducers of the virus to households during interpandemic outbreaks. 15, 16, 19 Neuzil et al. 14 found excess rates of absenteeism from work among parents, who missed almost 1 day of work for every 3 days of school missed because of influenza. This calculation included only missed working days to care for a sick child, and not working days that might have been missed if the parent also caught influenza; therefore it is reasonable to assume that the actual rate of parental absenteeism is even higher.
The role of influenza in increasing absenteeism from day care or school in otherwise healthy children is also indirectly supported by our data on the impact of influenza vaccination on children, which are in line with previous estimates. 13, 17, 20 A blinded placebo-controlled study comparing two influenza vaccines (an inactivated split-virus vaccine and a live attenuated, cold-adapted vaccine) in 555 schoolchildren in Russia found that both vaccines reduced school absenteeism by 47 to 56%. 20 The unvaccinated children had an excess of 79 missed school days. 20
The role of children in the transmission of influenza in the community is also indirectly confirmed by the studies that analyzed the impact on households of vaccinating children against influenza. In a randomized controlled trial of influenza vaccine in preschool children, the rate of febrile respiratory illnesses was 42% lower in unvaccinated household contacts of influenza-vaccinated children than in those living with unvaccinated children. 21 Data from Tecumseh, MI, 22 and Japan 23 also indicate that mass vaccination of school age children is associated with reduced respiratory illnesses and all-cause mortality in the community. These experiences support the economic modeling of influenza immunization programs 24–27 and suggest that large scale immunization of children can affect community epidemics.
In conclusion influenza in otherwise healthy children has a considerable impact, especially socioeconomically. Vaccinating these children might significantly reduce the direct and indirect costs of influenza also on their household contacts. Our results encourage a wider use of influenza vaccine in healthy children of all ages to reduce the burden of influenza in the community.
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