Disclosures: The authors report no relevant financial disclosures.
June 03, 2021
2 min read

CDC unable to estimate flu vaccine effectiveness after historically mild season

Disclosures: The authors report no relevant financial disclosures.
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The United States experienced “unusually low” influenza activity this past season, likely as a result of the various measures taken to curb the spread of COVID-19, including mask wearing, social distancing and reduced travel.

The season was so mild, in fact, that there are not enough available data to estimate how effective the 2020-2021 influenza vaccines were, Timothy M. Uyeki, MD, MPH, MPP, chief medical officer in the CDC’s Influenza Division, told Healio.

flu shot
The United States saw unusually low influenza activity in 2020-2021.
Source: Adobe Stock.

In a viewpoint published recently in JAMA, Uyeki and colleagues noted that the rate of laboratory-confirmed influenza hospitalizations this season — 0.8 per 100,000 people as of April 30 — is the lowest since at least 2005, the first year such data were collected.

Uyeki and colleagues wrote that the effect has “highlighted the potential for similar prevention efforts, combined with influenza vaccination, to control severe influenza epidemics and pandemics.”

“Continuing use of properly fitted face masks for health care personnel, patients, and visitors can help to reduce influenza virus transmission in health care settings,” they wrote. “Most influenza virus transmission is thought to occur from symptomatic persons, but more research is needed on the contribution of asymptomatic and presymptomatic transmission, and the effectiveness of face masks to reduce influenza virus transmission in public settings,” they wrote.

The transmission of some respiratory viruses, such as respiratory syncytial virus, has already begun to increase as a result of the loosening of COVID-19 mitigation measures, Uyeki told Healio. However, he said the extent to which transmission increases depends on other factors, including whether sick people stay at home, ventilation is sufficient indoors and how many people congregate and continue to wear face masks.

He said transmission will likely be lower outdoors — as temperatures and humidity increase — but that influenza circulation may increase in the fall and winter if nonpharmaceutical interventions are not followed.

“Expanding influenza vaccination and other preventive measures will be very important for the coming fall and winter seasons,” he said.

Because of the unpredictability of influenza, Uyeki said it is uncertain whether the 2021-2022 influenza season will be more severe than this past season. A large number of people may be more susceptible to influenza because of “waning immunity” from less exposure to the virus since early 2020, he said.

“We are always monitoring influenza activity in the Southern Hemisphere and worldwide and will be particularly interested in the extent of influenza virus circulation, what influenza virus strains are circulating, and the severity of the approaching season in the Southern Hemisphere,” Uyeki said.

The FDA’s Vaccines and Related Biological Products Advisory Committee voted unanimously in March to adopt WHO’s influenza vaccine component recommendations for next season, which included changes to the vaccine viruses for influenza A(H1N1) and A(H3N2) from last season. Uyeki said it is also unclear how the reduced circulation of influenza viruses and the lower number of viruses that were analyzed will affect the match between circulating influenza viruses and the new vaccines.

“Since influenza viruses are continuously evolving, it is always important to closely monitor the characteristics of circulating viruses by influenza virus surveillance in the U.S. and worldwide year-round,” he said.