Feb. 25, 2019 -- This year's flu season hasn't been as severe as last year's, but it may not have peaked yet, public health officials say. So, if you dutifully got your flu shot in the fall, will your immunity wane before the current flu season does?

Maybe, say vaccine experts, depending on your age, the type of vaccination you got, and how bad the season gets.

The question of waning vaccine immunity has been under study recently, with some experts finding immunity does decline. It’s difficult to find clear-cut answers as to how much, because each flu season -- and each annual vaccine -- come with so many variables.

If you get the flu vaccine say in August, antibody levels drift down, and by the end of the season, ''you might be more at risk, but it's remarkably difficult to prove,” says Ann Falsey, MD, a professor of medicine at the University of Rochester School of Medicine in New York, who studies respiratory viruses.

What Research Finds

The risk of getting the flu rises about 16% every 28 days after vaccination, according to a study by Kaiser Permanente Northern California researchers. They looked at flu seasons from 2010 through 2017 and the medical records of nearly 45,000 people who tested positive for flu.

Researchers placed subjects into three categories: People in Group A tested positive for the flu 14 to 41 days after being vaccinated; In Group B, people got the flu 42 to 69 days after receiving the vaccine; and people in Group C were vaccinated more than 154 days before getting the flu.

Patients in Group B were 1.3 times more likely to have gotten the flu than those in Group A. Patients in Group C were twice as likely to have gotten the flu, compared with those in Group A.

Other researchers turned to data from the U.S. Influenza Vaccine Effectiveness Network, using information from the 2011 through 2015 seasons. They found that vaccine effectiveness declined by about 7% a month for influenza A (H3N2) and influenza B, and about 6% to 11% for influenza A (H1N1).

The CDC has published a recommendations report, including timing of the flu vaccine, says Kristen Nordlund, a spokesperson.

Even though it’s possible that your vaccine protection will wane, the CDC recommends getting the shot by the end of October.

Studies have not found that one age group sees vaccine protection waning more quickly than others, the CDC says, nor has the strain of flu virus mattered.

Looking at older adults, Melissa Stockwell, MD, an associate professor of pediatrics and of population and family health at Columbia University, says ''we don't know enough about the waning in a season'' to make hard and fast recommendations at the beginning of the season. The researchers concluded there may be benefits to vaccinating older patients as close to the start of flu activity to maximize protection.

She also does not recommend that adults get a second flu shot.

This Year's Vaccine and Season

So, could vaccine waning be to blame for a late-season surge in flu?

"It's a really good question, but I don't know if we could ever really know the answer," says Laura Haynes, PhD, a professor of immunology at the UConn Center on Aging at the University of Connecticut School of Medicine, Farmington.

"This is the middle of winter, this is when flu should be peaking," she says. "It might also have to do with weather," she says of cases rising. "In the Northeast, we had a pretty mild January, and people were outside. In February, we got snow, and people were inside more. The flu is transmitted when people are [crowded] together and humidity is low. People are breathing on each other and touching doorknobs."

More Recommendations

This season, if you haven't gotten vaccinated, it is not too late, Stockwell says.

As for next season? If you are over 65, she suggests talking to your doctor about the high-dose or adjuvant vaccines, meant to boost the immune response in older adults.

For others, the advice to get the flu shot by the mid- to late October is still good, Stockwell says. "It can take 2 weeks to reach full protection against the flu [for adults]. And some years, flu season begins as early as November."

Show Sources

Melissa Stockwell, MD, associate professor of pediatrics, Columbia University Vagelos College of Physicians and Surgeons; associate professor of population and family health, Columbia University Mailman School of Public Health.

Kristen Nordlund, CDC spokesperson.

Ann Falsey, MD, professor of medicine and infectious diseases, University of Rochester School of Medicine. 

CDC: "Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices -- United States, 2018-19 Influenza Season," Aug. 24, 2018.

Vaccine: “Within-season influenza vaccine waning suggests potential net benefits to delayed vaccination in older adults in the United States.”

Clinical Infectious Diseases: “Intraseason Waning of Influenza Vaccine Effectiveness.”

Clinical Infectious Diseases: “Intraseason waning of influenza vaccine protection: Evidence from the US Influenza Vaccine Effectiveness Network, 2011-12 through 2014-15.”

Laura Haynes, PhD, professor of immunology, UConn Center on Aging, University of Connecticut School of Medicine, Farmington.


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