The burden of the influenza A H1N1pdm09 virus since the 2009 pandemic

In the spring of 2009, a novel influenza A (H1N1) virus emerged. It was detected first in the United States and spread quickly across the United States and the world. This new H1N1 virus contained a unique combination of influenza genes not previously identified in animals or people. This virus was designated as influenza A (H1N1)pdm09 virus. From April 12, 2009 to April 10, 2010, CDC estimated that there were 60.8 million cases (range: 43.3-89.3 million), 274,304 hospitalizations (195,086-402,719), and 12,469 deaths (8,868-18,306) in the United States due to the (H1N1)pdm09 virus.*

Since the introduction of the influenza A H1N1pdm09 virus in 2009, H1N1pdm09 has circulated seasonally in the U.S. causing illnesses, hospitalizations, and deaths.

The burden of influenza A H1N1pdm09 virus from 2009 – 2018

Limitations

These estimates of the cumulative burden of influenza A H1N1pdm09 virus from 2009 – 2018 are subject to several limitations.

We use multipliers to estimate the national burden of influenza from routine surveillance data. Data to calculate the multipliers often lag by two years; thus, we use multipliers measured during the 2009 pandemic and soon after. Any changes in testing practices, care-seeking behavior, or disease severity that occurred after the pandemic would not be reflected in the multipliers.

Frequently Asked Questions

What does the cumulative burden of burden of influenza A H1N1pdm09 virus from 2009 – 2018 mean?

The cumulative burden of burden of influenza A H1N1pdm09 virus from 2009 – 2018 is an estimate of the number of people who have been sick, seen a healthcare provider, been hospitalized, or died as a result of influenza A H1N1pdm09 since it first began spreading in humans in 2009. CDC does not know the exact number of people who have been sick and affected by influenza because influenza is not a reportable disease in most areas of the United States. Therefore, these numbers are estimated using a mathematical model, based on observed rates of laboratory-confirmed influenza-associated hospitalizations.

How does CDC estimate the cumulative burden of influenza A H1N1pdm09?

The methods CDC used to estimate the cumulative burden of influenza A H1N1pdm09 are based on methods that have been detailed elsewhere and are available at https://www.cdc.gov/flu/about/burden/estimates.htm [3, 15]. The cumulative burden included illnesses related to influenza A H1N1pdm09 viruses that occurred during the 2009 pandemic through the 2017–2018 influenza season.

The method uses mathematical multipliers to calculate illnesses, medical visits, and deaths from data on hospitalized cases reported through the hospital-based surveillance during the 2009 pandemic and Influenza Hospitalization Surveillance Network (FluSurv-NET). We further divided the burden to illnesses due to influenza A H1N1pd09 using the proportion of outpatients and inpatients infected with influenza A H1N1pdm09; the number of community illnesses and medically-attended illnesses were divided based on influenza A H1N1pdm09 frequencies from the US Flu VE Network and the number of hospitalizations and deaths were based divided based on H1N1pmd09 frequencies from FluSurv-NET. Of note, because data on influenza A subtype was missing for 60% of FluSurv-NET patients with influenza A virus infection, we used multiple imputation (70 imputations) to estimate the rate of hospitalization for each subtype, including patient age, surveillance site, and admission time period (October-December, January, February, or March-May) in the imputation model.

What do these estimates tell us about the public health impact of influenza?

These estimates are an example of the lasting impact and burden that can occur as a result of the introduction of a new influenza virus into humans. The impact of a novel influenza virus with pandemic potential can have significant and severe impact when it is first introduced into a population, because there is no previous exposure or immunity to the virus in the population. As we saw after the 2009 pandemic, a new influenza virus can continue to circulate, resulting in substantial burden of disease.