New Estimates on the Benefits of Flu Vaccination from the 2015-2016 Season

For the past few years, CDC has used a model to estimate the impact of annual influenza vaccinations. New data on estimates of flu illness, medical visits, and hospitalizations prevented by the 2015-2016 flu vaccines will be released, as well as a new range for deaths attributed to flu.

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New Estimates on the Benefits of Flu Vaccination from the 2015-2016 Season

Estimated Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths Averted by Vaccination in the United States

Introduction:

This web page provides estimates on the impact of annual influenza vaccination and the burden of influenza in the United States for the 2015-2016 influenza season, and will be updated annually.

For the past several years, CDC has used a model to estimate the numbers of influenza illnesses, medical visits, and hospitalizations, and the impact of influenza vaccination on these numbers in the United States (1-5). The methods used to calculate the estimates have been described previously (1,2) and are outlined briefly below. CDC uses the estimates of the burden of influenza in the population and the impact of influenza vaccination in influenza-related communications. Annual estimates on the number of influenza-related illnesses, medical visits, and hospitalizations, will be used to derive a five-year range to characterize the influenza burden in the United States. This range will be updated every five years.

Additionally, this web page provides estimates of influenza deaths and deaths averted by influenza vaccination. CDC calculates estimated influenza deaths in two ways: 1) using reports of pneumonia & influenza (P&I) deaths and 2) using reports of respiratory & circulatory (R&C) deaths attributable to influenza. P&I deaths are now available in real-time, while data on R&C deaths are available after a three-year delay. While both estimates are useful, P&I deaths represent only a fraction of the total number of deaths from influenza because they do not capture the deaths that occurred among people not tested for influenza or deaths that result from respiratory and cardiovascular complications. Calculations based on R&C deaths are used in CDC influenza-related communications materials because these calculations provide a more complete estimate of the actual burden of influenza. CDC will continue to present the mortality burden of influenza as a range, rather than a median or average, to better reflect the variability of influenza and will update estimates of R&C deaths as the data become available.

2015-2016 Estimates:

For the 2015-2016 influenza season, CDC estimates that influenza vaccination prevented approximately 5.1 million influenza illnesses, 2.5 million influenza-associated medical visits, and 71,000 influenza-associated hospitalizations (see Table 1). These estimates of averted disease burden are comparable to some previous seasons (see Table 2). During the 2015-2016 influenza season, CDC estimates that influenza vaccination prevented 3,000 P&I deaths (see Table 1). This estimate is similar to estimates of averted P&I deaths during previous seasons (see Table 2). Past comparative data suggest that for the 2015-2016 season the total number of influenza-associated R&C deaths prevented by vaccination may be between two and four times greater than estimates using only P&I deaths (see Table 2).

To calculate these estimates, CDC used 2015-2016 estimates of influenza vaccination coverage (32.2% to 69.7%, depending on age group), vaccine effectiveness (24% to 57%, depending on age group), and influenza hospitalizations rates (20.3 per 100,000 to 321.1 per 100,000, depending on age group and adjusted for influenza testing and test sensitivity).

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2015-2016 Discussion:

During the 2015-2016 season, influenza A (H3N2) viruses circulated early in the season but influenza A (H1N1)pdm09 viruses predominated overall (6). The season was notable because influenza activity peaked in mid-March, 2016; one of the later peaks on record. In the United States influenza activity most commonly peaks between December and February. The overall burden of influenza was substantial with an estimated 25 million influenza illnesses, 11 million influenza-associated medical visits, 310,000 influenza-related hospitalizations, and 12,000 P&I deaths (see Table 3). (Note that past comparative data suggest that the total number of influenza-associated R&C deaths may be between two and four times greater than estimates using only P&I deaths). Overall, the burden estimates for last season are lower than the estimated burden for the three previous seasons, but are near the middle of the range for the previous five seasons (see Table 4).

While influenza seasons can vary in severity, during most seasons people 65 years and older experience the greatest burden of severe influenza disease. This was also true for the 2015-2016 season. While people in this age group accounted for only 15% of the U.S. population, they made up 50% of influenza-associated hospitalizations and 64% of P&I deaths during the 2015-2016 season. Influenza vaccination is the best way to prevent influenza virus infection, and among adults 65 years and older CDC estimates that vaccination prevented 23% of influenza-related hospitalizations during the 2015-2016 season. Vaccine coverage dropped by about 3 percentage points in this age group (to about 63%) between the 2014-2015 and 2015-2016 influenza seasons (7). Such drops in influenza vaccination coverage are costly for older adults, who are at high risk of serious influenza-related complications. If, instead of declining, vaccine coverage had increased by just 5 percentage point in this age group, an additional 36,000 illnesses and more than 3,000 additional hospitalizations could have been prevented during the 2015-2016 season.

The fraction of averted illness from vaccination was lowest among the broader range of working-age adults, aged 18 to 64 years, owing to low vaccination coverage in general in this age group, a drop in vaccine coverage among people 50 to 64 years old, and lower vaccine effectiveness among people 50 to 64 years during the 2015-2016 season (see Table 5). With more than 16 million illnesses from influenza estimated last season and vaccine coverage estimated at 36%, increasing vaccination coverage among persons 18 to 64 years old could have a large impact on reducing illness and work absenteeism. Specifically, if vaccination coverage had increased by 5 percentage points among adults aged 18 to 64 years during the 2015-2016 season, 300,000 additional influenza illnesses and 2,000 additional hospitalizations could have been prevented.

If vaccination rates increased by just 5 percentage points across the entire population, another 500,000 influenza illnesses, 230,000 influenza-associated medical visits, and 6,000 influenza-associated hospitalizations could be prevented. If vaccination rates improved to the Healthy People goal of 70 percent for all age groups, another 2.4 million influenza illnesses and 19,000 influenza-associated hospitalizations could have been prevented. Similarly, improvements to vaccine effectiveness could provide incremental public health benefit.

Strategies to improve vaccine coverage in all ages include ensuring influenza vaccination status is assessed at each heath care encounter during the influenza season (October through May), ensuring that everyone 6 months and older receive a recommendation to get vaccinated and an offer of vaccination from their provider, using standing orders in the health care office, using patient reminder/recall systems aided by immunization information systems, and expanding vaccination access through use of nontraditional settings for vaccination (e.g., pharmacies, workplaces, and schools) to reach persons who might not visit a physician’s office during the influenza season.

Conclusion:

Influenza vaccination during the 2015-2016 influenza season prevented an estimated 5.1 million illnesses, 2.5 million medical visits, 71,000 hospitalizations, and 3,000 P&I deaths. (Note that past comparative data suggest that the total number of influenza-associated R&C deaths prevented by vaccination may be between two and four times greater than estimates using only P&I deaths). Efforts to increase vaccination coverage will further reduce the burden of influenza, especially among working-age adults younger than 65 years, who continue to have the lowest influenza vaccination coverage. This report underscores the benefits of the current vaccination program, but also highlights areas where improvements in vaccine uptake and vaccine effectiveness could deliver even greater benefits to the public’s health.

Although the timing and intensity of influenza virus circulation for the 2016-2017 season cannot be predicted, peak weeks of influenza activity have occurred between December and February during about 75% of seasons over the past 30 years, and significant circulation of influenza viruses can occur as late as May. Therefore, vaccination should be offered to anyone aged ≥6 months by the end of October if possible and for as long as influenza viruses continue to circulate.

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Limitations:

These estimates are subject to several limitations. First, influenza vaccination coverage estimates were derived from reports by survey respondents, not vaccination records, and are subject to recall bias. Second, these coverage estimates are based on telephone surveys with relatively low response rates; nonresponse bias may remain after weighting. Estimates of the number of persons vaccinated based on these survey data have often exceeded the actual number of doses distributed, indicating that coverage estimates used in this report may overestimate the numbers of illnesses and hospitalizations averted by vaccination. Third, this model only calculates outcomes directly averted among persons who were vaccinated. If there is indirect protection from decreased exposure to infectious persons in a partially vaccinated population (i.e., herd immunity), the model would underestimate the number of illnesses and hospitalizations prevented by vaccination. Fourth, vaccine effectiveness among adults 65 years and older might continue to decrease with age, reaching very low levels among the oldest adults who also have the highest rates of influenza vaccination; thus, the model might have overestimated the effect in this group. Fifth, due to data availability, we are unable to estimate influenza-associated R&C deaths for the 2014-2015 or 2015-2016 seasons. P&I deaths are a fraction of all deaths associated with influenza. Based on past studies, (8,9) the total number of R&C deaths associated with influenza that occurred during the 2015-2016 season may be two to four times higher than reported P&I deaths. As data on R&C deaths become available CDC will update estimates of influenza-associated deaths (see CDC Study: Flu Vaccine Saved 40,000 Lives During 9 Year Period).

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Previous Estimates:

Previous estimates of the burden of illness, medical visits, and hospitalizations related to influenza are available online and in scientific publications. (1-5) Estimates using the same methodology as for the 2015-2016 season are shown in the tables below to provide context for the current season’s estimates.

The estimates of P&I deaths related to influenza are a fraction of all deaths related to influenza that occurred a given season. Data on the number of R&C deaths are available with a three-year lag and, therefore, are available for the 2010-2011 through 2013-2014 influenza season. Using these data, CDC estimates that influenza-associated R&C deaths have ranged from a low of 12,000 (during 2011-2012) to a high of 56,000 (during 2012-2013).

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2015-2016 Tables for Influenza Burden and Burden-Averted Estimates:

Table 1: Estimated number and fraction of influenza illnesses, medical visits, hospitalizations, and pneumonia and influenza deaths averted by vaccination, by age group — United States, 2015-2016 influenza season