The Advisory Committee on Immunization Practices recommends annual influenza vaccination
for all persons aged ≥6 months to reduce morbidity and mortality caused by influenza
in the United States (1). CDC previously developed a model to estimate that annual
influenza vaccination resulted in 1.1–6.6 million fewer cases and 7,700–79,000 fewer
hospitalizations per season during the 2005–2013 influenza seasons (2,3). For the
2013–14 influenza season, using updated estimates of vaccination coverage, vaccine
effectiveness, and influenza hospitalizations, CDC estimates that influenza vaccination
prevented approximately 7.2 million illnesses, 3.1 million medically attended illnesses,
and 90,000 hospitalizations associated with influenza. Similar to prior seasons, fewer
than half of persons aged ≥6 months are estimated to have been vaccinated.* If influenza
vaccination levels had reached the Healthy People 2020 target of 70%, an estimated
additional 5.9 million illnesses, 2.3 million medically attended illnesses, and 42,000
hospitalizations associated with influenza might have been averted. For the nation
to more fully benefit from influenza vaccines, more effort is needed to reach the
Healthy People 2020 target.
The methods used have been described in detail previously (2) and are outlined briefly
in this report for the 2013–14 season. First, CDC estimated the number of illnesses,
medically attended illnesses, and hospitalizations associated with influenza that
occurred in the United States during the 2013–14 influenza season. Laboratory-confirmed
influenza-associated hospitalization rates by age group were obtained from FluSurv-NET,
a collaboration between CDC, the Emerging Infections Program Network, and selected
health departments in 13 geographically distributed areas in the United States that
conduct population-based surveillance.† Hospitalization rates were adjusted for underreporting
based on the frequency and sensitivity of influenza testing in surveillance hospitals
during two post-pandemic seasons (4); hospitalization rates were multiplied by a factor
of 2.1 for ages <20 years, 3.2 for 20–64 years, and 5.3 for ≥65 years. In previous
years, influenza hospitalization rates were multiplied by a factor of 2.7 based on
data collected during the 2009 influenza pandemic that were not age-specific (2,3).
Data were collected during two post-pandemic seasons to update these multipliers (4)
because influenza testing might not be as common as during the pandemic and the previous
multipliers might have underestimated hospitalizations in nonpandemic years. The updated
multipliers were similar to the previous estimates for children and younger adults,
but indicate that estimated hospitalization rates among older adults in recent seasons
were too low.
Adjusted rates were applied to the U.S. population by age group to calculate numbers
of hospitalizations. The numbers of influenza illnesses were estimated from hospitalizations
based on previously measured multipliers that reflect the estimated number of ill
persons per hospitalization in each age group: 143.4 for 0–4 years; 364.7 for 5–19
years; 148.2 for 20–64 years, and 11.0 for ≥65 years (2). The numbers of persons seeking
medical care for influenza were then calculated using age group-specific data on the
percentages of persons with a respiratory illness who sought medical attention, which
were estimated from results of the 2010 Behavioral Risk Factor Surveillance Survey:
67% for ages 0–4 years; 51% for ages 5–19 years; 37% for ages 20–64 years, and 56%
for ages ≥65 years (2).
Second, 2013–14 estimates of vaccination coverage through April 2014 and end-of-season
vaccine effectiveness data were used to estimate how many persons were not protected
by vaccination during the season and thus were at risk for influenza illness, medically
attended illness, and influenza-related hospitalization. The rate of each outcome
among persons at risk was then used to estimate the number of influenza-associated
outcomes that would have been expected in the same population if no one had been protected
by vaccination. Estimates of 2013–14 influenza vaccination coverage were based on
self-report or parental report of vaccination status using data from the National
Immunization Survey for children aged 6 months–17 years and Behavioral Risk Factor
Surveillance Survey data for adults aged ≥18 years, and varied from 37% to 70%, depending
on the age group (Table 1) (5). Vaccine effectiveness estimates for the 2013–14 season
were derived from the U.S. Influenza Vaccine Effectiveness Network, a group of five
academic institutions that conduct annual vaccine effectiveness studies (5,6). The
network estimates the effectiveness of vaccination for preventing real-time reverse
transcription polymerase chain reaction–positive influenza among persons with acute
respiratory illness of ≤7 days duration seen in outpatient clinics in communities
in five states. Vaccine effectiveness estimates were updated to include data collected
through the end of season and ranged from 39% (95% confidence interval [CI] = −6%–65%)
for persons aged ≥65 years to 56% (CI = 37%–69%) for persons aged 5–19 years (Influenza
Division, National Center for Immunization and Respiratory Diseases, CDC; unpublished
data; 2014).
Finally, the averted outcomes attributable to vaccination were calculated as the difference
between outcomes in the hypothetical unvaccinated population and the observed vaccinated
population. Calculations were stratified by month of the year to account for annual
variations in the timing of disease and vaccination and then summed across the whole
season. The prevented fraction was calculated as the number of averted illnesses divided
by the total illnesses that would have been expected in an unvaccinated population.
During October 2013–May 2014, influenza vaccination resulted in an estimated 7.2 million
(CI = 5.1–9.9) fewer illnesses, 3.1 million (CI = 2.1–4.4) fewer medically attended
illnesses, and 90,068 (CI = 51,231–144,571) fewer hospitalizations (Table 2) associated
with influenza. Overall, 16.9% (CI = 15.3%–18.0%) of these adverse health outcomes
associated with influenza were prevented. Using the same model, if vaccination levels
had instead reached the Healthy People 2020 target of 70%,§ an additional 5.9 million
illnesses, 2.3 million medically attended illnesses, and 42,000 hospitalizations might
have been averted.
Although 17% of the averted illnesses and 24% of averted medically attended illnesses
were among children aged 6 months–4 years and persons aged ≥65 years (two groups known
to be at higher risk for complications), these two age groups accounted for 60% of
averted hospitalizations. Persons aged ≥65 years accounted for 55% of all hospitalizations
prevented.
Discussion
During the 2013–14 season, influenza activity peaked in late December, and the influenza
A (H1N1)pdm09 virus predominated in the United States for the first time since the
2009 pandemic (7). There were somewhat fewer estimated influenza-associated hospitalizations
overall than during the previous season (3), which had been a moderately severe season
during which influenza A (H3N2) viruses predominated. In 2013–14, however, rates of
hospitalization for adults aged 20–64 years were 1.3–5.5 times higher than during
previous reported seasons (2,3). In addition, 109 influenza-associated pediatric deaths
(deaths among persons aged <18 years) were reported to CDC (7), most of which were
associated with influenza A (H1N1)pdm09.
During the 2013–14 season, a 17% overall reduction in illnesses resulted in a large
number of prevented influenza-associated medical visits and hospitalizations. The
prevented fraction was similar to recent seasons (2,3) and was highest among children
aged <5 years (25%) and lowest for adults aged 20–64 years (15%). Fewer than half
of adults aged 20–64 in the United States are vaccinated each season despite a recommendation
for universal influenza vaccination for persons aged ≥6 months (1). Adults aged 20–64
years make up approximately 60% of the U.S. population and during the 2013–14 season
accounted for 77% of estimated influenza illnesses and 46% of hospitalizations (Table
1). This sizeable population has the lowest influenza vaccination coverage (37%) and
therefore the most potential gains through use of strategies known to improve coverage.
Such strategies include ensuring that all those who visit a health care provider during
the influenza season receive an influenza vaccination recommendation from their provider,
using patient reminder/recall systems, using immunization information systems, and
expanding 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.¶
What is already known on this topic?
Influenza vaccination has been a central tool for influenza prevention in the United
States for more than 50 years. Previously, CDC estimated that annual influenza vaccination
resulted in 1.1–6.6 million fewer cases and 7,700–79,000 fewer hospitalizations annually
during the 2005–2013 influenza seasons.
What is added by this report?
Using surveillance data, vaccination coverage survey data, and vaccine effectiveness
estimates collected during the 2013–14 season, estimates of the impact of influenza
vaccination for the 2013–14 season were generated. Vaccination during the 2013–14
season resulted in an estimated 7.2 million fewer cases of influenza, 90,000 fewer
hospitalizations, and 3.1 million fewer medically attended cases than would have been
expected without vaccination. If vaccination levels had reached the Healthy People
2020 target of 70%, an additional 5.9 million illnesses, 2.3 million medically attended
illnesses, and 42,000 hospitalizations might have been averted.
What are the implications for public health practice?
Although influenza vaccination prevented millions of illnesses and tens of thousands
of hospitalizations in 2013–14, there is a need for increased vaccination coverage
and more effective vaccines to further reduce the burden of influenza.
During 2013–14, the vaccine effectiveness point estimate was lowest among persons
aged ≥65 years (39% [CI = −6%–65%]). Almost half of the 2013–14 estimated hospitalizations
(Table 1), and in many years >90% of influenza deaths (9), occur among adults aged
≥65 years. A recent study using this same analytic framework showed that even with
very low vaccine effectiveness among older adults (10%), current influenza vaccination
coverage in older adults can still help to prevent a sizeable number of influenza
hospitalizations during moderately severe seasons (8). Vaccination coverage rates
are relatively high in this vulnerable population; therefore, major gains in preventing
severe outcomes in this age group will require vaccines with better efficacy for persons
in this age group.
The findings in this report are subject to at least five limitations. First, influenza
vaccination coverage estimates were derived from reports by survey respondents, not
vaccination records, and are subject to recall bias. Furthermore, these estimates
are based on telephone surveys with relatively low response rates; although weighting
adjustments were designed to improve representativeness of the sample, they might
not completely eliminate nonresponse bias. Estimates of the number of persons vaccinated
based on these survey data have exceeded the actual number of doses distributed, indicating
that coverage estimates might be somewhat lower than those used in this report and
might overestimate the numbers of illnesses and hospitalizations averted by vaccination.
Second, this model only calculates outcomes directly averted among persons who were
vaccinated. If there is indirect protection from decreased exposure of unvaccinated
persons 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. Third, vaccine effectiveness was lower for adults aged ≥65 years;
the effectiveness might continue to decrease with age, reaching very low levels among
the oldest adults with the highest rates of influenza vaccination; thus, the model
might have overestimated the effect in this group. Fourth, this model assumes that
vaccine effectiveness is the same for all outcomes. Finally, the fraction of persons
with influenza who seek medical care was estimated from data collected during the
2009 pandemic, although the values were similar to those derived from surveys conducted
the following season (10). If health care seeking differed during the 2013–14 influenza
season, the number of influenza medical visits in the population might have been overestimated
or underestimated.
Influenza vaccination prevented a substantial amount of influenza disease in the United
States last season, including an estimated 3 million medical visits and 90,000 hospitalizations.
Although vaccines with increased effectiveness are needed, much can be done to maximize
influenza prevention during the upcoming 2014–15 season. In particular, efforts to
increase vaccination coverage will further reduce the burden of influenza, especially
among adults aged 20–64 years, who continue to have the lowest influenza vaccination
coverage. Although the timing and intensity of influenza virus circulation for the
2014–15 season cannot be predicted, peak weeks of influenza activity have occurred
in January through March in >75% of seasons during the past 30 years, and significant
circulation can occur as late as May. Therefore, vaccination should continue to be
offered through the peak periods of influenza virus circulation and as long as influenza
viruses are reported to be circulating for the current season.