The US Food and Drug Administration’s (FDA’s) Vaccines and Related Biological Products Advisory Committee has chosen the influenza vaccine strains for the 2022–2023 season in the northern hemisphere, which beginnings in the fall of 2022.
On March 3, the committee unanimously voted to endorse the World Health Organization’s (WHO’s) recommendations as to which influenza strains to include for coverage by vaccines for the upcoming flu season. Two of the four recommended strains are different from last season.
The committee also heard updates on flu activity this season. So far, data from the US Flu Vaccine Effectiveness (VE) network, which consists of seven study sites, have not shown that the vaccine is protective against influenza A. “We can say that it is not highly effective,” said Brendan Flannery, PhD, who leads the US Flu VE network for the Centers for Disease Control and Prevention (CDC), in an interview with Medscape Medical News. He was not involved with the advisory committee meeting. Flu activity this season has been low, he explained, so there are fewer cases his team can use to estimate vaccine efficacy. “If there’s some benefit, it’s hard for us to show that now,” he said.
The panel voted to include a A/Darwin/9/2021-like strain for the H3N2 component of the vaccine; this is changed from A/Cambodia/e0826360/2020. For the influenza B Victoria lineage component, the committee voted to include a B/Austria/1359417/2021-like virus, a swap from this year’s B/Washington/02/2019-like virus. These changes apply to the egg-based, cell-culture, and recombinant vaccines. Both new strains were included in WHO’s 2022 influenza vaccine strain recommendations for the southern hemisphere.
For the influenza A H1N1 component, the group also agreed to include a A/Victoria/2570/2019 (H1N1) pdm09-like virus for the egg-based vaccine and the A/Wisconsin/588/2019 (H1N1) pdm09-like virus for cell culture or recombinant vaccines. These strains were included for the 2021–2022 season. The panel also voted for the inclusion of a B/Phuket/3073/2013-like virus (B/Yamagata lineage) as the second influenza B strain for the quadrivalent egg-based, cell culture, or recombinant vaccines, which is unchanged from this flu season.
“Sporadic” Flu Activity
While there was an uptick in influenza activity this year compared to the 2020–2021 season, hospitalization rates are lower than in the four seasons preceding the pandemic (from 2016–2017 to 2019–2020). As of February 26, the cumulative hospitalization rate for this flu season was 5.2 hospitalizations per 100,000 individuals. There have been eight pediatric deaths due to influenza so far this season, compared to one pediatric death reported to the CDC during the 2020–2021 flu season.
About 4.1% of specimens tested at clinical laboratories were positive for flu. Since October 30, 2.7% of specimens have been positive for influenza this season. Nearly all viruses detected (97.7%) have been influenza A.
Lisa Grohskopf, MD, MPH, a medical officer in the influenza division at the CDC who presented the data at the meeting, described flu activity this season as “sporadic” and noted that activity is increasing in some areas of the country. According to CDC’s weekly influenza surveillance report, most states had minimal influenza-like illness (ILI) activity, although Arkansas, Idaho, Iowa, Kansas, Minnesota, and Utah had slightly higher ILI activity as of February 26. Champaign-Urbana, Illinois; St. Cloud, Minnesota; and Brownwood, Texas, had the highest levels of flu activity in the country.
Low Vaccine Effectiveness
As of January 22, results from the US Flu VE network do not show statistically significant evidence that the flu vaccine is effective. Currently, the vaccine is estimated to be 8% effective against preventing influenza A infection (95% CI, -31% to 36%) and 14% effective against preventing A/H3N2 infection (95% CI, -28% to 43%) for people aged 6 months and older.
The network did not have enough data to provide age-specific VE estimates or estimates of effectiveness against influenza B. This could be due to low flu activity relative to prepandemic years, Flannery said. Of the 2758 individuals enrolled in the VE flu network this season, just 147 (5%) tested positive for the flu this season. This is the lowest positivity rate observed in the Flu VE network participants with respiratory illness over the past 10 flu seasons, Grohskopf noted. In comparison, estimates from the 2019 –2020 season included 4112 individuals, and 1060 tested positive for flu.
“We are really at the bare minimum of what we can use for a flu vaccine effectiveness estimate,” Flannery said about the more recent data. The network was not able to produce any estimates about flu vaccine effectiveness for the 2020–2021 season because of historically low flu activity.
The Department of Defense also presented vaccine efficacy estimates for the 2021–2022 season. The vaccine has been 36% effective (95% CI, 28% – 44%) against all strains of the virus, 33% effective against influenza A (95% CI, 24% – 41%), 32% effective against A/H3N2 (95% CI, 3% – 53%), and 59% effective against influenza B (95% CI, 42% – 71%). These results are from a young, healthy adult population, Lieutenant Commander Courtney Gustin, DrPH, MSN, told the panel, and they may not be reflective of efficacy rates across all age groups.
Though these findings suggest there is low to no measurable benefit against influenza A, Flannery said the CDC still recommends getting the flu vaccine, as it can be protective against other circulating flu strains. “We have been able to demonstrate protection against other H3 [viruses], B viruses, and H1 viruses in the past,” he said. And as these results only show protection against mild disease, “there is still possibility that there’s benefit against more severe disease,” he added. Studies measuring effectiveness against more severe outcomes are not yet available.
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