Public health agencies and pharmacies start pushing annual influenza vaccination in late summer, but a new study suggests that might be too early for some older adults.

The report in the American Journal of Preventive Medicine suggests that having older adults wait until October to get their flu shots could avoid tens of thousands of influenza cases and hundreds of deaths.

The University of Pittsburgh School of Medicine–led analysis points out that, based on previous research, the protection offered by the flu vaccine wanes as the season progresses an estimated 6% to 11% per month. That suggests that delaying immunization until closer to the start of flu season might create greater immunity, the researchers write.

On the other hand, the study team notes, if flu season arrives early or if delayed vaccination prompts more than one in 20 people who would otherwise be vaccinated to skip their flu shot, then the gains are erased.

“There’s controversy in the public health community over whether influenza vaccination should happen as soon as the vaccine becomes available in August, or if it’s better to wait until later in the fall,” explained lead author Kenneth J. Smith, MD, MS, professor of medicine and clinical and translational science in Pitt’s Division of General Internal Medicine. “What we’ve found is that it's a balancing act, but if a clinician believes a patient will return for vaccination in the fall, then our analysis shows that it is best if they advise that patient to wait.”

The results were based on computer models that contrasted a “compressed” vaccination period that begins in October to the current situation, in which the vaccine becomes available in August. The focus was on people aged 65 years or older, who are more at risk for influenza and its complications because their immune systems don't typically mount as strong a defense to infections as younger people. The study also points out that older adults tend to have higher early vaccination rates than younger adults.

Researchers used a Markov model to compare influenza likelihood in older adults with 1) status quo vaccination (August–May) to maximize vaccine uptake or 2) vaccination compressed to October–May (to decrease waning vaccine effectiveness impact).

Data from the from the 2013–2014 and 2014–2015 flu seasons were collected and analyzed in 2018.

Results indicate that compressed vaccination would avert 11,400 or more influenza cases in older adults during a typical season, if it does not decrease vaccine uptake, and avoid more than 250 deaths. If compressed vaccination decreases vaccine uptake or there is an early-season influenza peak, however, more influenza could result. The authors calculated that compressed vaccination was never favored if it decreased absolute vaccine uptake by more than 5.5% in any scenario; at the same time, when influenza peaked early, status quo vaccination was favored.

“Compressed vaccination could decrease waning vaccine effectiveness and decrease influenza cases in older adults,” researchers explain. “However, this positive effect is negated when early season influenza peaks occur and diminished by decreased vaccine uptake that could occur with shortening the vaccination season.”

“In all scenarios, simply getting vaccinated is the best way to avoid the flu,” Smith said. “If the choice is between getting the influenza immunization early or not getting it at all, getting it early is definitely better.”

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