Experts recommend that pregnant women get a flu shot each year, and now a new study suggests that the earlier they do it, the better.

The study, which estimated the effects of flu vaccination at different points in the flu season, found that the benefits wane if vaccination is pushed past November.

Vaccinations later in the season didn’t prevent as many doctor visits and hospitalizations, researchers report in the American Journal of Obstetrics and Gynecology.

“The message is, the earlier you get vaccinated, the better,” said Dr. Evan R. Myers of Duke University Medical Center, who led the study.

In the U.S., the flu season can start in October and last until May, but it usually peaks between January and March.

Ideally, yearly vaccination should begin in September, or as soon as that season’s flu vaccine is available, according to the Centers for Disease Control and Prevention.

While the flu usually causes no more than a week or so of misery, pregnant women are at higher-than-average risk of severe symptoms that can land them in the hospital.

Women who will be in their third trimester during the peak of flu season are at particularly high risk.

However, flu shots are also important for women who will give birth early in the season or just before, Myers said. Those women won’t be at high risk of complications during the flu season - but their infants will.

Infants younger than 6 months cannot be vaccinated against the flu, Myers said, but if a mother gets her shot during pregnancy, some of her protective antibodies against the infection will be passed to her baby.

For their study, Myers and his colleagues used a mathematical model to estimate the benefits of flu vaccination, and different timings of vaccination, during pregnancy.

They found that compared with no vaccination, giving flu shots to all pregnant women would prevent more than one-third of their hospitalizations for the flu - cutting the number to 1,235 in a year.

It would also trim the annual number of doctor visits for the flu made by pregnant women, from more than 54,000 per year to about 23,000.

Infants younger than 6 months would benefit, too. Their hospitalizations for the flu would fall from more than 4,700 per year, to just over 3,000, the researchers estimate.

However, the greatest benefits were seen when vaccination was done sooner - November or earlier, Myers said.

That was especially true when it came to preventing infants’ illnesses: the later vaccination was put off, the more babies who would be unprotected during the peak of flu season.

“A lot of the benefit is in preventing disease in the baby,” Myers told Reuters Health. “I believe that’s something we should be emphasizing more.”

He noted that for every 10 pregnant women in the U.S., fewer than 4 get a seasonal flu shot each year.

It’s not clear whether fears about vaccine safety are behind that low rate, according to Myers. But one thing that might worry some women, he said, is the fact that some flu vaccine formulations contain thimerosal, a mercury-containing preservative.

There’s no evidence that the small amounts of thimerosal in the vaccine are harmful. But for women who are worried, Myers said, thimerosal-free flu shots are available.

A recent government study also found no evidence of unusual complications from flu vaccination in U.S. pregnant women over the past 20 years.

Among the most common side effects were skin reactions at the injection site, symptoms like fever and fatigue, and mild allergic reactions. There was no evidence linking the vaccine to an increased risk of miscarriage or stillbirth.

The current study was funded by GlaxoSmithKline Inc., one of several companies that make the seasonal flu vaccine.

Some insurance companies will cover the cost of a flu shot. In other cases, employers set up flu vaccination programs for their employees. Many people choose to get the shots - often for around $25 - at large retail chains like Costco, K-mart, and Walmart, or at drugstore chains like Walgreens and CVS, for example.

SOURCE: American Journal of Obstetrics & Gynecology, online April 18, 2011.