Disclosures: Ferdinands reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.
February 09, 2021
2 min read

Flu vaccine effectiveness declines around 10% per month following shot

Disclosures: Ferdinands reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.
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Influenza vaccine-provided immunity declines by around 10% each month following vaccination, according to study results published in Clinical Infectious Diseases. Researchers said September or October are the best months to get the shot.

“An annual influenza vaccine is recommended for everyone 6 months and older in the United States. However, the best time to get an annual flu vaccine remains up for debate,” Jill M. Ferdinands, PhD, an epidemiologist in the CDC’s Influenza Division, told Healio. “With mounting evidence showing that vaccine-induced immunity wanes over the course of the flu season, it’s important to consider whether early vaccination — for example, in July or August — may result in reduced immune protection against flu before the end of the season between March and May, especially among older adults.”

flu shot
Researchers observed a “significant” waning of influenza vaccine-provided immunity of between 8% and 9% each month following vaccination.
Credit: Adobe Stock

According to Ferdinands, current recommendations attempt to balance the need to immunize the population before flu starts to circulate each year with concerns about waning immunity by recommending that vaccination be offered by the end of October.

“This study hoped to gain a better understanding of the extent of in-season waning of influenza vaccine protection to help reduce uncertainty regarding the best time to get a flu vaccine,” she said.

Ferdinands and colleagues assessed vaccine efficacy among adults enrolled in the U.S. Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) who were hospitalized for acute respiratory illness during the 2015-2016 through 2018-2019 influenza seasons. According to the study, participants were interviewed to collect information about demographics, vaccination and illness characteristics while respiratory specimens were tested for influenza. Patients who tested positive for influenza were conted as cases, and those testing negative counted as controls. The researchers then used multivariate logistic regression models with influenza-associated hospitalization as the outcome and time in days between vaccination and symptom onset as the predictor to estimate vaccine efficacy.

Overall, the study demonstrated that influenza vaccine effectiveness peaks shortly after vaccination and then declines at a rate of 8% per 30 days against influenza A (H3N2), 9% per 30 days against influenza A (H1N1), and 8% per 30 days against influenza B viruses among all adults 18 years or older.

Ferdinands noted that, among adults 65 years and older, the declines are even greater, with effectiveness declining at a rate of 11% per 30 days against influenza A (H3N2), 10% per 30 days against influenza A (H1N1), and 11% against influenza B viruses.

“The study found that there is significant, in-season waning of vaccine-provided immunity against flu. The public health implications of these findings warrant closer examination because optimizing timing of influenza vaccination could lead to appreciable gains in public health benefits, given the large burden of influenza morbidity and mortality annually in the U.S.,” Ferdinands said.

“Getting vaccinated in July or August is probably too early, especially for adults 65 years and older, because of the likelihood of reduced protection against flu infection later in the flu season,” she said. “For now, September and October are good times to get vaccinated for most people.”