A new finding of a possible flu shot-miscarriage link shows science prioritizes vaccine safety

When I first heard about the study published in the journal Vaccine on Tuesday that linked getting a flu shot to having a spontaneous abortion (or what is more commonly called miscarriage) over the course of the next 28 days, I knew it was likely to make waves.

Few things today can evoke as much concern, if not downright panic, as declaring that doing something that is routinely recommended, like getting your annual flu shot, may be harmful, like causing a pregnant woman to have a miscarriage. Yet while this story may be potentially bad news for newly pregnant women who get a flu shot, it is also a good news story about science, showing that the science community is doing its job, painstakingly watching for any signals that a recommended intervention might be harmful to health.

The researchers, from the Marshfield Clinic, Kaiser Permanente, Group Health Research Institute, and the Centers for Disease Control and Prevention, emphasized that the results may be just a chance finding, something that occurs when you do a number of studies. It’s like flipping a coin five times: Three times out of 100 you can expect all five flips to result in heads. That is a rare finding, but it’s due to chance alone, not to some causal effect.

But there is a scientific reason to think that the new findings may be a true signal we need to take seriously. While multiple studies in the past have found that women who received flu vaccine during pregnancy did not have a higher rate of miscarriage than women not vaccinated, this is the first study to focus on three important variables: 1) vaccines that contained the 2009 H1N1 flu virus strain, the virus that caused the 2009 influenza pandemic; 2) the 28 days after vaccination; and 3) the impact of repeat flu vaccination over two flu seasons.

The Vaccine study found that women who had a miscarriage were twice as likely to have been vaccinated within the previous 28 days as women with full-term pregnancies. An additional analysis showed that women who had a miscarriage were almost eight times more likely to have been vaccinated with H1N1-containing vaccines in both the previous year and the year of the miscarriage than pregnant women who did not have miscarriages.

The new H1N1 virus may have some unique characteristics that make it more likely to cause miscarriage. It is already known that this strain can cause serious health problems in pregnant women. During the 2009 H1N1 pandemic, 5 percent of all flu-related deaths occurred in pregnant women, even though they represented less than 1 percent of the population.

What, then, is a woman to do this flu season? What should health care professionals recommend? The CDC and its expert public health panel, the Advisory Committee on Immunization Practices, continues to recommend flu shots for pregnant women during any trimester of pregnancy. Flu infection and pregnancy is a potentially dangerous mix. But this is where the good news comes in.

The public health community, a champion of vaccination, has over the past several decades clearly documented the remarkable reduction in vaccine-preventable infections and deaths, primarily in children. We are also the champions of vaccine safety, and so have initiated numerous studies and disease surveillance programs to look carefully for any signals of potential problems with vaccines.

The new study published in Vaccine used data from the CDC-supported Vaccine Safety Datalink, a network established in 1990 to monitor vaccine safety. A second study to evaluate the risk of H1N1-containing vaccines causing miscarriage in early pregnancy, funded by the CDC, is now in progress using more recent data from the Vaccine Safety Datalink data.

Most Americans aren’t aware that public health agencies and health care organizations are constantly monitoring vaccine safety, never resting on previously conducted studies as proof of safety. This point is often lost on, or ignored by, those who oppose routine childhood immunizations based on flawed data analysis and conclusions.

The study published Tuesday is a compelling example of how the public health and medical communities value vaccine safety as a top priority. Unfortunately, it will take more research to definitively determine if the connection between flu vaccine and miscarriage is real and, if so, what changes are needed for recommendations about getting vaccinated against the flu.

Those studies are being conducted by dedicated health professionals who strongly support vaccines as essential in preventing infectious diseases, but who also strongly believe in evidence. Science is often referred to as “self-correcting” because the collection of new information informs what we know and do. Only science-supported facts can give us the truth.

Knowing that these safety activities are ongoing is why I so confidentially and strongly support the recommended vaccination schedules for Americans.

Michael T. Osterholm, Ph.D., is a regents professor of public health and director of the Center for Infectious Disease Research and Policy at the University of Minnesota and adjunct professor at the University of Minnesota Medical School.