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Illustration: Aïda Amer/Axios

A growing number of anecdotes about COVID-19 vaccines affecting a person's menstrual cycle is spurring attention and research funding.

Why it matters: Efforts to halt the pandemic are being stymied by continued vaccine hesitancy, in part due to disinformation about side effects. A CDC scientist tells Axios "there is absolutely no evidence" that the altered periods reported by some are causing infertility, a common refrain among anti-vaxxers.

  • "Women of childbearing age should absolutely be vaccinated," says CDC medical officer Christine Olson, who is head of the v-safe pregnancy registry.

Threat level: Pregnant people who have a symptomatic COVID infection "have a twofold risk of admission into intensive care and a 70% increased risk of maternal death," particularly with the highly transmissible Delta variant, Olson says.

What's happening: There are "thousands and thousands and thousands" who are reporting an impact from the COVID vaccine, particularly on menstruation, says Namandjé N. Bumpus, director of the department of pharmacology and molecular sciences at Johns Hopkins University School of Medicine.

  • However, Bumpus adds, "it remains to be seen if there's a causal link or not."
  • Olson points out that the CDC "is aware of reports of menstrual irregularities" and continues studying the issue. NIH is funding five one-year supplementary grants worth $1.67 million to further investigate if there's a link and what the underlying mechanisms may be.
  • One of the theories is that the change in a person's period could be caused by a temporary immune response to the vaccine, as the endometrium lining the uterus contains multiple immune cells, says Alice Lu-Culligan, an M.D.-Ph.D. student at Yale School of Medicine who is researching the topic and wrote an opinion piece in April.

Changes in menstrual cycles can be caused by many factors like stress or new medications, can happen frequently, and often are temporary without an impact on fertility, says Viki Male, a lecturer in reproductive immunology at Imperial College London.

  • The problem, she says, is the combination of the infertility misinformation with people saying, "Oh, I noticed it changed my period." This has led to hesitation and questions like: "You say they don't cause infertility but if they don't cause infertility, then why has my friend's period come five days late?"
  • "But people find [menstruation] goes back to normal very quickly," Male says.
  • "Alteration of someone's period does not mean they were infertile that month," Lu-Culligan adds.

Background: Misinformation about the vaccine's effect on fertility was first promulgated in the U.K. in December by Michael Yeadon, a former Pfizer vice president, Male says. Since then, many different scientists have looked closely at the allegation, she says, and found no evidence to support that claim.

  • "The concern about infertility is hypothetical. There is no current evidence to support this, and there are most certainly pregnancies occurring every day following COVID-19 vaccination," Olson says.

Between the lines: Multiple sources say the decision in the early clinical vaccine trials to exclude pregnant people fueled hesitancy.

  • "It was a dreadful mistake to not include people who are pregnant in clinical trials" for COVID-19 vaccines at first, Male says. "We should have known that we were going to need to vaccinate people who are pregnant, but we ended up without trial data from them. I think that's something we should also learn from in the future."
  • "A lot of that early opportunity was lost" by not including pregnant people in the vaccine randomized control trials with a blinded placebo cohort, Lu-Culligan says. "There are trials going on right now, but we have to wait for those and wait for the numbers to accumulate."

The bottom line: Vaccination is essential "for anyone wanting to make sure their pregnancy is the healthiest it can be and results in a healthy infant," Olson says.

Go deeper

Nov 19, 2021 - Podcasts

The latest on COVID booster shots

The FDA is likely to sign off shortly on the Pfizer booster shot for everyone 18 and over, but many cities and states like New York City and California have already made boosters available to all adults. We look at what we do and don't know about who should get boosters and what kind is best.

  • Plus, President Biden meets with his North American counterparts.
  • And, the growing global reach of Mexican food.

Guests: Dr. Namandje N. Bumpus, Director of the Department of Pharmacology and Molecular Sciences at Johns Hopkins Medicine; and Axios' Dave Lawler and Russell Contreras.

Credits: Axios Today is produced in partnership with Pushkin Industries. The team includes Niala Boodhoo, Sara Kehaulani Goo, Julia Redpath, Alexandra Botti, Nuria Marquez Martinez, Alex Sugiura, Sabeena Singhani, Lydia McMullen-Laird, David Toledo and Jayk Cherry. Music is composed by Evan Viola. You can reach us at podcasts@axios.com. You can text questions, comments and story ideas to Niala as a text or voice memo to 202-918-4893.

Go deeper:

Updated 15 hours ago - Politics & Policy

Omicron dashboard

Illustration: Aïda Amer/Axios

  1. Health: CDC prepares tougher testing rules for international travelers — U.S. on the lookout for Omicron casesFDA panel backs Merck's antiviral COVID pill.
  2. Politics: Biden says fight against Omicron won't include "shutdowns or lockdowns" — Two federal judges temporarily block Biden vaccine mandates.
  3. Vaccines: Omicron adds urgency to vaccinating world — Omicron fuels the case for COVID boosters — Pentagon denies Oklahoma National Guard request for exemption from vaccine mandate.
  4. World: Omicron variant detected in more countriesWHO advises people 60 or older to postpone travel due to Omicron — COVID-19 "radically altered mobility" globally, UN says.
  5. Variant tracker: Where different strains are spreading.
Updated 25 mins ago - Sports

MLB enters first lockout since '95 as deal expires

Baseball Commissioner Rob Manfred (L) and Major League Baseball Players Association executive director Tony Clark. Photo: Matt King/MLB via Getty Images

Major League Baseball's collective bargaining agreement expired at 11:59 p.m. ET Wednesday without a new deal in place.

Why it matters: With no CBA, the MLB is in a management lockout — the first work stoppage since a 1994-95 strike led to the cancelation of the World Series for the first time in 90 years.