Does the COVID vaccine make your breasts bigger?

Some women who received the Pfizer or Moderna COVID-19 vaccines have reported side effects that resemble breast cancer symptoms. In addition to experiencing disproportionately severe side effects compared to men, along with abnormal menstrual cycles, doctors have also seen female patients develop swollen lymph nodes after receiving the vaccine. This has some people speculating that the COVID vaccine can make your breasts larger, dubbing it the “Pfizer boob job.”

Swollen lymph nodes can be an early sign of cancer—particularly when the swelling occurs in the armpit on one side of the body—but there are other reasons behind the condition, such as when the body is fighting off a cold or an infection. 

Instances of swollen lymph nodes following a COVID-19 vaccination have a relatively straightforward explanation, one which thankfully doesn’t involve cancer.

The mRNA vaccines deliver a small amount of genetic code to instruct cells how to replicate the virus’ surface protein “spike,” which in turn activates the immune system. Lymph nodes contain B-cells that use this information to generate antibodies in response to a foreign entity. The buildup of antibodies in the lymph nodes may result in them becoming enlarged. This can lead to breasts feeling larger for a short time following a COVID vaccine, but it is not cause for undue concern.

The Centers for Disease Control and Prevention documented 11% of patients with swollen lymph nodes after their first shot of the Moderna vaccine. Sixteen percent of patients experienced swollen nodes following their second shot. The swelling typically occurs two to four days after injection and can last between four and 12 weeks. 

Some doctors have expressed concern that women who experience swollen breasts as a side effect of the COVID vaccine may undergo unnecessary diagnostic tests to check for breast cancer.

“It is important to recognize that lymph nodes may become enlarged for a number of reasons having nothing to do with malignant disease, including in patients known to have cancer,” Dr. Maurie Markman, president of Medicine and Science at Cancer Treatment Centers of America, wrote in a blog post. “If someone is concerned, it is always reasonable to check with a physician.”

But in at least one case, the swollen lymph node side effect caused by the vaccine reminded a woman to check the rest of her body. She discovered she actually did have breast cancer.

The American Cancer Society and the Society of Breast Imaging are among several public health agencies that recommend women planning to get a mammogram do so before receiving the vaccine. If they must get screened after a vaccine, experts advise waiting between four and 12 weeks.

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