Routine mammograms should be done either before the first dose of a COVID-19 vaccine or four-to-six weeks after the second dose, the Society for Breast Imaging advises.

The vaccines' temporary side effects can include swollen lymph nodes around the armpits, which could be misread as a possible sign of breast cancer if it turns up on a mammogram. So-called axillary lymphadenopathy is usually seen in only 0.02%-0.04% of screening mammograms, according to the Society's guidelines.

In trials of the Moderna vaccine, the condition developed in 11.6% of participants after the first dose and in 16% after the second dose.

Researchers testing the Pfizer/BioNTech vaccine did not routinely ask participants about armpit tenderness and lymph node swelling, but some people reported this side effect, which lasted an average of 10 days. More subtle effects on lymph nodes that are evident only on X-rays are likely to last longer, the Society said, although it is not clear yet what vaccination-related lymph node changes would look like.

"As more information about the incidence and appearance of axillary lymphadenopathy following COVID-19 vaccination becomes available, it may be appropriate to change the duration of follow-up or final assessment recommendations," the Society said.