Estimated Influenza Illnesses, Medical visits, and Hospitalizations Averted by Vaccination in the United States — 2019–2020 Influenza Season

This web page provides estimates on the burden of influenza and the effects of annual influenza vaccination in the United States for the 2019–2020 influenza season.

For the past several years, CDC has used a model to estimate the numbers of influenza illnesses, medical visits, hospitalizations, and deaths, and to estimate the effects that influenza vaccination had on outcomes in the United States (1-6). The methods used to calculate the estimates have been described previously (1, 2, 6) and are outlined briefly below. CDC uses the estimates of the impact of influenza vaccination to inform policy and communications promoting influenza vaccination as the best way to prevent influenza.

2019–2020 Estimates

CDC estimates that during the 2019–2020 season 38 million people were ill, 18 million people went to a health care provider, 400,000 were hospitalized, and 22,000 died with influenza (7). The season was characterized by two consecutive waves of activity, beginning with influenza B viruses and followed by A(H1N1)pdm09 viruses. Overall, influenza A(H1N1)pdm09 viruses were the most commonly reported influenza viruses this season. The number of influenza-associated illnesses, medically attended illnesses, hospitalizations, and deaths were lower than some more recent seasons and similar to other seasons during which influenza A(H1N1)pdm09 viruses dominated (8).

Influenza vaccination prevented an estimated 7.52 million illnesses, 3.69 million medical visits, 105,000 hospitalizations, and 6,300 deaths due to influenza during the 2019-2020 season. The number and proportion of influenza-associated hospitalizations prevented by vaccination during 2019–2020 varied by age group, due to age-specific differences in influenza burden, vaccine coverage, and vaccine effectiveness. Vaccination prevented the lowest proportion of illnesses among adults aged 18 to 49 years, the age group in which vaccination coverage is lowest, and among children aged 5-17 years, for whom vaccine effectiveness was low for both influenza B and influenza A(H1N1)pdm09 viruses. Vaccination prevented the greatest proportion of outcomes among children aged 6 months to 4 years, an age group in which there was high vaccine uptake and the vaccine effectiveness was greatest.

Influenza vaccination is the best way to protect against influenza and associated complications; however, more effective or longer lasting influenza vaccines are needed. Several efforts are currently underway to understand and investigate factors that contribute to reduced or increased vaccine effectiveness and inform changes to vaccine composition, formulations, or production (9-11).

Improvements in vaccine coverage could also provide a greater public health benefit, producing even greater reductions in illnesses and demands for resources from the health system. During influenza season, the health care system, including outpatient clinics, emergency departments, and hospitals, experience high patient volumes. Any reduction in influenza-associated medical visits and hospitalizations will reduce demands on the healthcare system stressed by the COVID-19 pandemic and improve healthcare providers’ abilities to provide the best care. Strategies to improve influenza vaccine coverage include: using patient reminder/recall systems aided by immunization information systems; expanding access to vaccination at pharmacies, workplaces, and schools; and organizing mobile and/or outdoor vaccination clinics that would allow for social distancing among patients and healthcare providers.

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