In its early estimate of this season's flu vaccine effectiveness (VE) in the United States, the Centers for Disease Control and Prevention (CDC) said VE was modest—at 47%—but the vaccine afforded better protection in children but less in adults age 50 and older.
Midway through and at the end of each flu season the CDC and its collaborators at five study sites publish their flu vaccine protection estimates, based on findings from children and adults seen for acute respiratory infections at medical centers. The new update appears today in Morbidity and Mortality Weekly Report (MMWR).
The early look comes when flu activity is still increasing across the United States during a season dominated in all regions but the Southeast by 2009 H1N1, a more reliable foe for the vaccine compared with more complex challenges when H3N2 is the main strain.
In a separate MMWR report today on the US flu season so far, CDC scientists said severity as yet has been low, in contrast to the 2017-18 season, which was severe, much of it attributed to H3N2.
Findings influenced by H1N1 dominance
The interim VE estimate includes data from 3,254 children and adults seen at the five US Influenza Vaccine Effective Network sites from Nov 23, 2018, to Feb 2. Overall effectiveness against influenza was 47% (95% confidence interval [CI], 34% to 57%).
For children age 6 months to 17 years, VE was 61% (95% CI, 44% to 73%). For adults age 50 and older, VE against all strains was 24% (95% CI, -15% to 51%) and against 2009 H1N1 was 8% (95% CI, -59% to 46%). VE was not statistically significant for either figure in that age-group.
Of subtyped influenza A samples, 74% were 2009 H1N1. The team estimated that flu VE against 2009 H1N1 was 46% (95% CI, 30% to 58%) across all ages.
Small sample size limits comparisons
The researchers said the interim estimate of 46% against 2009 H1N1 in all age-groups is similar to the 45% VE against H1N1 seen in the most recent season dominated by 2009 H1N1, which was 2015-16.
They added, however, that protection was lower than a meta-analysis of VE against 2009 H1N1 in the years since the 2010-11 season, lower than Canada's recent interim estimate this season of 72% against the strain, and lower than the 78% VE Australia saw against the H1N1 strain during its 2018 flu season.
"The reasons for these differences might include limited sample size caused by low attack rates in some age groups, geographic differences in circulating viruses, and genetic variation within virus subtypes," the group wrote.
Though a limited number of H3N2 viruses were detected, the early analysis found a VE of 44% (95% CI, 13% to 64%) against the strain.
The team included a few caveats about interpreting the early findings. For example, they said the sample sizes are smaller than recent interim estimates, resulting in wide confidence levels, especially in adults ages 50 and older for whom protection was not significant. They also note that the small sample size for the interim analysis, common in mild or late flu seasons, limited the number of age groups included in the analysis. For example, they weren't able to look at protection levels in people ages 65 and older.
Better vaccines needed
Michael Osterholm, PhD, MPH, director of the University of Minnesota's Center for Infectious Disease Research and Policy (CIDRAP), which publishes CIDRAP News, said he credits the CDC for doing the studies, important work that helps sort out flu vaccine challenges.
So far, protection against H1N1 looks comparable to last year, he said.
End-of-season estimates will, he hopes, shed more light on the vaccine's protection among older people, given the sparse data in the early report. But for now, Osterholm said the early glimpse confirms that the vaccine offers some protection in terms of decreased morbidity and mortality.
Also, he said today's report underscores the need for better flu vaccines. "These vaccines are surely not what we want for the future," he added.
In today's report, the CDC recommends that healthcare providers continue to administer influenza vaccine as long as flu is circulating and that the vaccine can still prevent illness, hospitalization, and death associated with currently flu strains and others that appear—such as influenza B—later in the season.
Season tracking with past 2009 H1N1 seasons
In the update on the US flu season to date, the CDC said its markers show that, as of Feb 2, flu severity has been low.
Hospitalization rates and flu-related deaths have been relatively low, consistent with what the CDC has observed in past seasons that were dominated by 2009 H1N1. According to early estimates, 155,000 to 186,000 people have been hospitalized for flu so far, and 9,600 to 15,900 deaths have been linked to the virus.
Lab tests to track how closely circulating strains match the vaccine strains show that most flu viruses are similar to the cell-culture grown-vaccine strains, but genetic diversity in 2009 H1N1 strains belonging to clade 6B.1 has increased, a sign of ongoing evolution. Tests are also finding an increasing proportion of circulating H3N2 viruses that are distinct from the vaccine component.
The CDC's forecasts hint that elevated flu activity in parts of the United States will continue for several more week.
Feb 15 MMWR report on early flu VE
Feb 15 MMWR report on US flu season