Infectious Disease

CDC cannot assess the effectiveness of the flu vaccine after a historically mild season

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The United States experienced “abnormally low” influenza activity last season, likely as a result of various measures taken to contain the spread of COVID-19, including wearing masks, social distancing and reduced travel.

In fact, the season was so mild that there isn’t enough data to estimate how effective the 2020-2021 flu vaccines were. Timothy M. Stop, MD, MPH, MPP, Chief medical officer in the CDC’s influenza division, said Healio.

Flu shot

The United States had unusually low levels of influenza activity in the 2020-2021 period.
Source: Adobe Stock.

In a recent position published in JAMA, Uyeki and colleagues found that the rate of laboratory-confirmed influenza hospital admissions this season – 0.8 per 100,000 people on April 30 – is the lowest since at least 2005, the first year of such Data was collected.

Uyeki and colleagues wrote that the effect “highlighted the potential for similar prevention efforts in combination with flu vaccination to control major influenza epidemics and pandemics”.

“Continued use of properly fitted face masks for health workers, patients and visitors can help reduce influenza virus transmission in the healthcare sector,” they wrote. “Most influenza virus transmission is believed to come from symptomatic individuals, but more research is needed on the contribution of asymptomatic and presymptomatic transmission and the effectiveness of face masks in reducing influenza virus transmission in public settings,” they wrote.

Transmission of some respiratory viruses, such as respiratory syncytial virus, has already increased due to the relaxation of COVID-19 containment measures, Uyeki told Healio. However, he said the extent to which transmission increases depends on other factors, including whether sick people stay at home, there is adequate indoor ventilation, and how many people gather and continue to wear face masks.

He said that outdoor transmission will likely be less – as temperatures and humidity rise – but that influenza circulation can increase in fall and winter if non-pharmaceutical measures are not followed.

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

Due to the unpredictability of influenza, Uyeki said it was uncertain whether the 2021-2022 influenza season will be more severe than last season. Large numbers of people could be more susceptible to influenza because immunity “waned” after lower exposure to the virus since early 2020, he said.

“We are constantly monitoring influenza activity in the southern hemisphere and around the world, and we will be particularly interested in the extent of influenza virus circulation, the influenza virus strains circulating and the severity of the upcoming season in the southern hemisphere,” said Uyeki.

The FDA’s Advisory Committee on Vaccines and Related Biological Products voted unanimously in March to adopt the WHO recommendations for the Influenza Vaccine Component for next season, the changes to the vaccine viruses for Influenza A (H1N1) and A (H3N2) from last season included. Uyeki said it was also unclear how the reduced circulation of influenza viruses and fewer viruses analyzed will affect the correspondence between circulating influenza viruses and the new vaccines.

“As influenza viruses continue to evolve, it is always important to closely monitor the properties of circulating viruses through influenza virus surveillance in the US and around the world year round,” he said.

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