Axillary lymphadenopathy is usually seen in only 0.02 to 0.04 percent of screening mammograms, according to the society’s guidelines. In trials of the Moderna vaccine, the condition developed in 11.6 percent of participants after the first dose and in 16 percent 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 on only X-rays will probably last longer, the society said, although it is unclear 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.