The Centers for Disease Control and Prevention unleashed a PR blitzkrieg this week to urge about 85 percent of Americans to get vaccinated against the flu. The agency expanded its recommendations to include yearly shots for all children ages 6 months to 18 years and also calls for pregnant women to be vaccinated, along with healthcare workers and those over 50. A spate of news stories with headlines like "Flu shots: What's your excuse?" and "Time for flu shots, now for 6-month-olds too" tells me that journalists swallowed the CDC's recommendations hook, line, and sinker. Where, though, is the worry about the fact that flu shots usually contain thimerosal, a mercury-containing compound that was banished from other childhood vaccines several years ago?
I'm particularly concerned about giving flu shots to pregnant women because no one has really identified the threshold dose for which thimerosal can become problematic for tiny fetuses. Certainly, the amount of mercury in a single flu shot is very small and most likely harmless. "It's equivalent to the amount in a small can of tuna fish," says Tom Clarkson, a mercury researcher at the University of Rochester. "Still, we know that high levels of mercury can affect cell division in the developing brain of a fetus, and no one can say with absolute certainty that there's no risk."
The other concern is that the kind of mercury found in vaccines is different from the methyl mercury found in fish. While thimerosal gets flushed from the body much faster than methyl mercury, what remains is more likely to accumulate in the brain, as inorganic mercury, and remain there for a year or more, according to a 2005 University of Washington study of infant monkeys. "We still don't have enough data to say how long this inorganic mercury stays in the brain, but if you can reduce or eliminate your baby's exposure, why wouldn't you do that?" says Tom Burbacher, a professor of environmental occupational health sciences who led the study.
The American College of Obstetricians and Gynecologists sent out a news release yesterday urging all pregnant women to get flu shots, with the proviso that shots containing thimerosal are fine because "there is no evidence showing that thimerosal is a danger to the health of the pregnant woman or her fetus." When I read that quote to Burbacher, he said, "that is a very misleading statement because women will assume there's been exhaustive research done on this showing that it's safe for fetuses, and there's no such data."
He doesn't mean that pregnant women should avoid getting flu shots. Instead, Burbacher says, they should get one that's thimerosal free. Yes, they're available, but you might need to ask around to find a doctor's office that dispenses them. Many don't, probably because not enough women are asking for them.
Why isn't the CDC recommending thimerosal-free vaccines to pregnant women? I posed this question to John Iskander, who is the acting director for immunization safety at the CDC. "From a public health standpoint, there's still not enough women receiving the flu vaccine, and we don't want to throw up another barrier in the vaccination process," he told me. But the individual pregnant woman who's concerned about unknown risks, Burbacher recommends, should ask for that flu vaccine without thimerosal.