Woman Receiving a VaccineAs Covid vaccine eligibility continues to grow, more and more people are getting up close and personal with its typical side effects: arm pain, fatigue, and mild fever. Recently, one of the lesser-known side effects of the Moderna and Pfizer vaccines has been causing concern, as it mimics a breast cancer symptom. The vaccines can cause swollen lymph nodes under the armpit, which can feel like a sore lump.

This side effect is a condition known as axillary adenopathy or enlarged lymph nodes in the armpit area. The vaccinations can cause changes in both the size and consistency of the lymph nodes in this area. Although this side effect is rarely reported after a flu shot or HPV shot, it is being reported at higher rates as a result of both Covid-19 vaccinations. The swelling can appear two to four days after vaccination and could last for a week or so.

These enlarged lymph nodes can be detected during mammographic screens. If you have been recently vaccinated and have an upcoming mammogram on your calendar, you should consider speaking to your doctor about whether or not to delay your screening. The Society of Breast Imaging recommends that women wait 4-6 weeks after vaccination to schedule their annual mammogram to ensure no false positives are detected.  Some medical professionals may advise you to wait up to a full three months after your final vaccination to undergo your mammogram.* If you chose to delay your mammogram during the pandemic, don’t forget to reschedule your appointment.

Although this side effect might be frightening, this should not dissuade you from scheduling your Covid-19 vaccination. If you have any further concerns about the vaccine and its side effects, it’s best that you discuss them with your doctor or trusted healthcare professional.

* Updated April 2022. Delaying mammograms after COVID vaccination is no longer recommended.