One common side effect of vaccination against COVID-19 closely resembles a possible symptom of metastatic cancer: enlarged lymph nodes near the armpit or collarbone.

There have been multiple cases of enlarged lymph nodes since the first doses were administered to health care workers and residents of long-term care facilities in mid-December, according to The New York Times. These can lead to cancer-related false positives, unneeded tests and anxiety for people living with cancer. While this swelling is both harmless and temporary (it typically passes within six weeks of vaccination, the Times reports), it can mimic a developing tumor in imaging tests.

Several medical journals have published articles on the subject. “This is especially relevant for patients with certain cancers, such as breast cancer, head and neck cancers, lymphoma and melanoma of the back and upper extremities, which have a predilection for metastasizing to these lymph node stations,” researchers at Memorial Sloan Kettering Cancer Center wrote in the Journal of Radiology last month. The enlarged lymph nodes may also show up as worrisome signs in women getting routine preventive mammograms.

The results of clinical trials indicate that about 11.6% of people can expect to experience lymph node swelling after the first dose of the Moderna vaccine; about 16% experience this effect after the second dose. The Pfizer-BioNTech vaccine can cause swelling too, but at a reduced incidence; the Johnson & Johnson vaccine does not appear to cause swelling at all. However, according to radiologists, current estimates are likely artificially low.

To avoid needless concern, experts recommend scheduling routine cancer screenings, such as mammograms, CT scans and MRI scans, no sooner than four to six weeks after vaccination. (They do not recommend delaying nonroutine screenings.)

People with a history of cancer and people with active cancer, many of whom are immunocompromised and eligible for early vaccination, may be particularly disturbed to find a prominent lump near vital organs.

“I am particularly eager to get the word out to all the patients undergoing surveillance after successful prior treatment of cancer,” Constance D. Lehman, MD, PhD, a professor of radiology at Harvard Medical School and the director of breast imaging at the Massachusetts General Hospital, told the Times. “I can’t imagine the anxiety of getting the scan and hearing ‘We found a node that is large. We don’t think it’s cancer but can’t tell’ or, worse, ‘We think it might be cancer.’”

These patient populations may be asked to provide a tissue sample to confirm that the swelling is caused by an immune response rather than the spread of cancer. But to spare themselves the agony of not knowing, Lehman said, they can opt to have the shot in the leg—where it will not have that effect—rather than the arm.

For more on how COVID-19 vaccination affects people with cancer, read “Are Coronavirus Vaccines Safe and Effective for People With Cancer?” And for more on how cancer and cancer treatment can put you at greater risk for severe COVID-19, read “The New Coronavirus and COVID-19” and “Fearing The Deadly Combo Of COVID-19 And Cancer.”