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The 2009 influenza pandemic

10 years later

Wiley, Sarah K. BSN, BA, RN

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doi: 10.1097/01.NURSE.0000615128.93339.22
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THE INFLUENZA A (H1N1)pdm09 virus, or “swine flu,” is a novel strain that circled the globe in 2009 and resulted in approximately 575,400 deaths.1 Emerging in Mexico, it made its way to the US and became headline news within weeks. The 2019 calendar year marked the pandemic's 10th anniversary.

As healthcare professionals, we should reflect on what we learned from the 2009 influenza pandemic and consider how this experience affects planning for future outbreaks of novel influenza strains. This article discusses the evolution of the 2009 swine flu pandemic, surveillance trends over the past decade, and the current state of preparedness for future pandemics.


The influenza virus is the eighth leading cause of mortality in the US.2 It can survive for up to 24 hours outside of the body and is transmitted between patients via coughing, sneezing, talking, or shaking hands.3 Although the virus is comprised of only eight genes, compared with an estimated 30,000 in humans, it has devastating effects on patients every year.4-6

Influenza is divided into types A, B, C, and D.7 It has an unstable, spherical structure and can infect, transmit, mutate, and even jump species.4-6 Types A and B are associated with seasonal outbreaks, with influenza B transmitted only between humans.7,8 Because it is found in both humans and animals, influenza A is the only type known to cause pandemics.5,7,9,10 Two surface proteins, hemagglutinin (H) and neuraminidase (N), denote how each strain is identified. There are 18 known subtypes of hemagglutinin and 11 known subtypes of neuraminidase, leading to numerous combinations such as H1N1, H3N2, and H5N1.7 (See Influenza pathology.)

In 2009, a strain of the H1N1 influenza A virus mutated and jumped species from swine to humans. When a new viral strain emerges, it is referred to as novel. Novel strains are significant because most humans, especially younger people, have no natural immunity to defend against it.

In the US, October marks the beginning of the annual flu season as primary care offices, clinics, and hospitals stockpile vaccines and face masks in preparation for the months ahead. Each year, healthcare professionals wonder if the coming season will introduce a new strain that will trigger a pandemic like the one in 2009.

Anatomy of a pandemic

In February 2009, an unidentified respiratory illness emerged in a 6-month-old Mexican girl.11 By mid-March, the illness had affected several people in Mexico City. The World Health Organization (WHO) soon got involved, but it was too late to contain the virus.

The virus reached the US on April 15, 2009, when it was detected in a pediatric patient in California. Within 10 days, it had spread to Texas, Kansas, Ohio, and New York.12 The strain was quickly identified and designated as the (H1N1)pdm09 virus, commonly referred to as 2009 H1N1. By June, over 1 million cases had been detected in all 50 states, Washington DC, Puerto Rico, and the US Virgin Islands. In weeks, it had made its way around the globe. WHO officially declared a pandemic on June 11, 2009.1 The CDC developed a vaccine that became available in December 2009.1

Between April 2009 and 2010, an estimated 151,700 to 575,400 deaths were attributed to 2009 H1N1 worldwide.7,13 Although typical flu seasons produce similar mortality, H1N1 was unusual in that 80% of these patients were under age 65.13

Unusual morbidity and mortality pattern

In evaluating the global burden of the 2009 H1N1 strain between 2009 and 2010, researchers derived mortality data from 12 countries. These included both lab-confirmed cases and H1N1-associated deaths from respiratory illness and cardiovascular disease. The wide range in estimated mortality can be attributed to patients who died from H1N1-related causes but were not tested for the disease.14

Typically, morbidity and mortality from influenza are higher in older adults compared with younger people. In the 2009 pandemic however, morbidity and mortality were higher among younger, previously healthy patients. Due to exposure to older H1N1 viruses earlier in their lives, many older adults may have had circulating H1N1 antibodies that provided some protection against the novel strain.

Patients over age 65 were less susceptible to the novel virus in part due to the genetic lineage of earlier influenza A strains from previous decades. It has also been theorized that patients born before the flu pandemics of 1957 and 1968 possessed cross-reactive antibodies to the 2009 strain, as portions of the H1N1 hemagglutinin protein were similar in structure and may have offered partial protection.15

Pregnant women were also at an increased risk for mortality.16 Between 2009 and 2010, this population made up 12% of all H1N1-related deaths, approximately 20 times higher than an average flu season.16 As they already fall within a younger and more susceptible age range, pregnant women were especially vulnerable due to physiologic changes to the cardiac and respiratory systems during pregnancy.17

A decade later

More than 10 years after the first case, 2009 H1N1 remains a predominant circulating strain of the influenza virus. To date, it has caused approximately 100.5 million illnesses, 936,000 hospitalizations, and 75,000 fatalities in the US.1

Table 2009
Table 2009:
H1N1 surveillance trends28-36

Every US vaccine has contained 2009 H1N1 since it emerged, but occurrences of the virus have waxed and waned over the past decade due to two key factors: vaccine effectiveness and coverage (see 2009 H1N1 surveillance trends).

Vaccine effectiveness plays a critical role in how well individuals defend against infection. If the strain used in the vaccine does not match the circulating virus, it is essentially useless. This makes patients more susceptible to full-blown infections and more likely to spread the virus.

Widespread vaccine coverage achieves herd immunity, in which the larger population becomes resistant, preventing the virus from mutating and protecting those who are vulnerable such as infants and immunocompromised patients. The fewer individuals vaccinated in an entire population, the higher the risk of infection, mutation, and transmission. This process is called antigenic drift and can be attributed to the annual development of new combination flu vaccines.18

The 2009 H1N1 strain follows a cyclical pattern in which it rapidly declines and suddenly spikes. One theory suggests that vaccine-mediated immunity to H1N1 may wane in just 2 years, leading to an influx of susceptible individuals every few years.19 As such, annual vaccination is crucial for disease control and prevention.

Another explanation could be low rates of annual vaccine coverage in younger adults between ages 18 and 49. Only 26% to 33% of these individuals are estimated to have received an annual H1N1 vaccine.20 Declining immunity in the most susceptible population for this strain explains its seasonal foothold. The solution is to reduce transmission rates through herd immunity by decreasing the number of vulnerable hosts and interrupting the chain of infection.

Pandemic preparation

Before the outbreak in 2009, it had been 40 years since the last influenza pandemic. Pandemic strains of the flu are difficult to predict, isolate, and fight. The CDC, the US Department of Health and Human Services, WHO, and the National Institutes of Health have each developed research and surveillance programs to track novel outbreaks of influenza. The detection and monitoring of circulating influenza strains in both humans and animals via gene-sequencing technology allows for the expedient identification of novel strains.1

Additionally, these organizations have improved vaccine efficacy and availability, introducing new strategies such as cell culturing and recombinant production, which were approved by the FDA in 2012 and 2013, respectively. These methods involve rapidly culturing, manufacturing, and stockpiling vaccines in an outbreak.21 Clinical trials for a universal influenza vaccine are underway, and new antiviral medications to manage flu symptoms and reduce complications are being tested.18 Apart from efforts in research and development, healthcare professionals and communities must also take measures to prevent the spread of the next novel flu strain (see Patient guidance).

Extra precautions must be taken in managing potentially infected patients. Those exhibiting signs and symptoms of influenza in the ED or waiting rooms should be provided a face mask, isolated from other patients by a minimum of 3 ft, and encouraged to wash their hands or use antimicrobial hand scrubs frequently.22

In hospital settings, airborne isolation protocols should begin immediately before confirmation with lab testing.23 The CDC estimates that rapid influenza detection can result in a false negative in up to 30% of cases when community prevalence is high, so repeat testing may be necessary to rule out a novel strain.24

Healthcare professionals should make sure they are vaccinated annually. They can also reduce the risk of transmission by limiting staffing and visitors to all patients with suspected infections and cohorting patients with suspected and confirmed infections. Although this may not protect individuals from becoming infected with a novel strain of the flu, it prevents them from becoming a reservoir host to circulating strains with the potential for further mutation and/or transmission.25

Looking ahead

H1N1 remains widespread, costing millions in healthcare and lost productivity. In the US, the estimated economic impact of seasonal influenza is $11.2 billion annually.26 The estimated costs of the next pandemic influenza strain are approximately $250 billion.22 Out-of-pocket, a single dose of the influenza vaccine costs between $31 and $60 per individual; most insurance plans cover it in full.27 Given the cost of hospitalization, copays, prescriptions, complications, and missed school or work, vaccinations represent the best defense against the influenza virus at this time.

Although far from a cure, advances in prevention and healthcare management offer hope. With chronic diseases on the rise, a growing number of individuals are vulnerable to complications from influenza, including pneumonia, sepsis, and death. A pandemic strain could be devastating, and year-round emphasis on preparedness and response is vital. Healthcare professionals must be proactive in educating patients and the public about preventive measures to reduce transmission and burden of another influenza pandemic.

Influenza pathology5

Influenza enters the body through the eyes, nose, or mouth. The hemagglutinin proteins attach to respiratory cells and, once the virus is absorbed, viral RNA is released. From there, influenza makes millions of copies of itself until neuraminidase forces the infected cell to explode, discharging all of the viral particles. The copying process is not perfect, however, and RNA is extremely unstable. As such, the copies often have mutations, which can change the structure of the hemagglutinin proteins before transmission, resulting in new viral strains not covered by vaccines.

Patient guidance5,22,25,37-40

Once a new strain has been identified, patients and families should be educated on the following:

  • recognizing early signs and symptoms that differentiate flu from the common cold, including fever, fatigue, chills, and myalgia, as well as gastrointestinal issues such as nausea, vomiting, diarrhea, and abdominal cramping.
  • contacting their healthcare provider for antiviral therapies. These drugs inhibit neuraminidase, the protein responsible for the release of viral particles after replication, and may lessen the duration of illness by 24 hours. Antiviral therapies work best within the first 48 hours of symptom onset, but they may still be helpful after this period. As the entire household may have been exposed, prophylaxis may also be beneficial for susceptible individuals.
  • staying home from school or work for at least 24 hours after the fever has resolved. If possible, patients should work from home until all symptoms subside, even if they are feeling better due to medication to manage signs and symptoms. Influenza is transmissible one day before symptom onset and remains contagious for up to one week.
  • cleaning and disinfecting all surfaces, especially frequently touched items such as keyboards, phones, doorknobs, toilets, and faucets, as influenza can stay active outside of the body for 24 hours.
  • no sharing of personal items such as utensils, water bottles, lip gloss or lip balm, and food.
  • frequent hand hygiene, especially after returning home from work or a public place.
  • respiratory hygiene and cough etiquette.
  • limiting travel between October and March, which is peak flu season. In the event of a pandemic, patients should postpone or cancel all travel.


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