Protection after early flu shots likely to last all season

Lisa Schnirring

Sep 29, 2009 (CIDRAP News) The current public health advice that Americans should get their seasonal flu shots early this year has prompted some to voice concern that their protection will fade later in the season. But medical literature and disease experts indicate there is no good evidence that immunity conferred by the shots wanes so quickly.

When the novel H1N1 virus emerged in the spring, pharmaceutical companies announced they would speed production of the seasonal shots to free up manufacturing capacity for the pandemic vaccine. Public health officials, anticipating the complex task of rolling out two flu vaccine campaigns this fall, have urged people to get the seasonal vaccine early to free up clinics to offer the pandemic H1N1 starting in mid October, when it is expected in large quantities.

National retail pharmacy chains such as Walgreens and CVS started offering seasonal flu vaccine in their stores on Sep 1. Seasonal flu vaccine campaigns don't usually begin until October.

Although this flu season may well be unusual because of the H1N1 virus, seasonal flu typically peaks in February or even latermore than 4 months from now. This has prompted questions as to whether the protection conferred by a flu shot received now will last through the season.

Anthony Fiore, MD, MPH, a medical epidemiologist in the CDC's Influenza Division, told CIDRAP News that there are no long-term data on how long protection lasts in younger populations. Though antibodies decline in the months following seasonal vaccination, it's not clear if the drop has any link to vaccine efficacy.

Seasonal flu vaccines are given each year because the viruses change, not because people have lost their antibody protection, he said. For example, he said some people who received the 1976 swine flu vaccine still show some evidence of immune system priming.

A February 2008 review of 14 studies on antibody persistence found no age-related declines in protection 4 or more months after flu vaccination. The authors noted that in the past, public health messages have raised unfounded concerns about waning protection over a season in elderly people who get flu shots. "Given the implications for the timing of seasonal and pandemic influenza immunization programs, historic statements expressing these concerns should be reconsidered," they wrote.

Fiore said there's no evidence base to suggest that waning immunity after vaccination during the flu season is a problem. He added that flu experts also believe cell-mediated immunity, which is difficult to measure, may also play a role in long-term protection.

He said the CDC isn't telling groups who usually plan to do their mid October seasonal flu vaccine clinics to move the dates up. "But the vaccine is here earlier and in large quantities, so there's no reason to wait."

"There's no full agreement in the clinical world when the best time to vaccinate is. It's a clinical management issue," Fiore said.

Experts also say the ebb and flow of seasonal flu this year can't be predicted. Thomas Frieden, MD, director of the US Centers for Disease Control and Prevention (CDC) said at a Sep 25 press conference that though the United States seems to be well into a new wave of pandemic H1N1 activity, officials aren't sure if the flu season will peak early and then decline for the rest of the fall and winter or if seasonal viruses will circulate at their normal times.

Frieden urged the public to take advantage of the early availability of the seasonal vaccine. "It's in large numbers. More than 50 million doses are in the system," he said. "That's more at this stage of the year than any previous year."

The US Department of Health and Human Services (HHS) has said it expects 116 million doses of the seasonal flu vaccine to be distributed to clinics and immunization sites. However, vaccine distribution problems have led to shortages in some parts of the country.

Meanwhile, seasonal flu vaccine campaigns are continuing in the United States, though several Canadian provinces have decided to postpone the seasonal shots except for seniors and those who have high-risk health conditions. Those decisions were based mainly on an unpublished Canadian study suggesting that people who received seasonal flu shots last season had a higher risk of pandemic H1N1 infectiona phenomenon that the CDC and World Health Organization have not confirmed.