The timing couldn’t be better - or worse - for flu shots this year. Pharmacies, medical offices, clinics and employers are all offering influenza vaccine now to help protect people from the flu whenever it strikes in a few months. As an Infectious Diseases Specialist, I believe that flu shots are important and can save lives, but what I heard on the news this morning worries me - a report potentially linking an increase in early miscarriages among certain women who received influenza vaccination. The finding is so preliminary and information so limited that it’s hard to draw any conclusions. So why am I worried? I’m concerned that many pregnant women who may have already been skeptical about receiving a vaccine will refuse a flu shot, placing them and their unborn children at increased risk for severe disease – even death. And the anti-vaccine movement may seize upon this report, creating panic and blowing it out of proportion to all of the evidence supporting the benefits of influenza vaccination, particularly in the most vulnerable populations (young children, the elderly, those with chronic illness, immune disorders, and pregnant and postpartum women) who are at higher risk for severe illness and complications from influenza.
The case-control study in question funded by the CDC and conducted from 2010 to 2012 matched 485 women who experienced miscarriages with 485 who delivered full-term. Those who had been vaccinated against influenza 28 days before the miscarriage, and in the first trimester of pregnancy, were more likely to have had a miscarriage if they had also been vaccinated the previous flu season with a vaccine containing the 2009 H1N1 virus (adjusted odds ratio of 7.7), suggesting that repeat vaccination may have led to a potential increase in the a proinflammatory response associated with the H1N1-containing vaccines. There was no association between miscarriage and flu vaccine if a woman had not received a vaccine in the previous year (aOR of 1.3) and no association seen in any other exposure window.The CDC is looking into this potential signal to determine if there is evidence for true “cause and effect” rather than just an association, so the finding is far from definitive. For example, it would be important to know whether these women had certain underlying medical conditions prompting them to seek annual vaccination that could have also placed them at increased risk for miscarriage. Only women who had clinically confirmed miscarriages were studied, so the proportion of women with clinically unrecognized pregnancy loss was uncertain. Results could have been biased if women who recognized and sought care for miscarriage were more likely to have been vaccinated in the 28-day exposure window.
Even though routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications, many people avoid getting vaccinated. The CDC recommends that all women who are pregnant or who might be pregnant in the influenza season receive influenza vaccine. This often occurs during the second and third trimesters, and prior published studies have shown that vaccination during pregnancy is not only safe, but beneficial to both the mother and the baby.
Despite this morning’s news and preliminary report, I hope physicians caring for pregnant women will “keep calm and vaccinate”!
Judith Wolf, MD
Associate Director, WHEP
Donahue et al. Association of spontaneous abortion with receipt of inactivated influenza vaccine containing H1N1pdm09 in 2010–11 and 2011–12. Vaccine. 2017; 35: 5314–5322 http://dx.doi.org/10.1016/j.vaccine.2017.06.069
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