This is your immune system at work, building up its defenses to COVID-19.
“Lymph nodes are primarily where your immune cells reside. They recognize an illness and then activate the immune system,” says Brian Brimmage, MD, an obstetrician who delivers babies at UNC REX Healthcare. “It’s normal after either a vaccination, an illness or infection for the lymph nodes to be enlarged because that’s where the big immune reaction is happening.”
Lymph nodes filter out harmful substances in your body, such as bacteria and viruses. They are located throughout your body, including in the neck, armpits and groin. They are activated—and thus become swollen—when they detect a harmful substance and try to remove it from your body.
The lymph nodes closest to the affected body part are usually the ones that swell. For example, if you have strep throat, the lymph nodes in your neck become swollen. If you get a sexually transmitted disease or vaginal infection, the lymph nodes in your groin might swell. Some people with breast cancer experience swollen lymph nodes in the armpit, which might cause anxiety after vaccination for women who don’t know to expect it.
Why would the vaccines cause swollen lymph nodes?
Because the COVID-19 vaccines are given in your arm, they can activate the lymph nodes in your armpit and cause swelling there.
Here’s why: The aim of the COVID-19 vaccine is to get your immune system to launch a response to what it thinks is the COVID-19 virus. It is training your body to make antibodies to fight the virus.
Then, if you’re exposed to COVID-19, those antibodies are ready to recognize the virus, resulting in either no infection or a less serious infection. Experiencing mild side effects after a vaccination, such as a slight fever, body aches and swollen lymph nodes, is your immune system telling you that it’s doing exactly what you want it to do—respond to the vaccine and produce antibodies. (That said, if you don’t notice side effects, don’t worry. The vaccine is still working in your body.)
“The reason that you are getting swelling in your armpit after the COVID vaccine is because your body is working really hard to mount an immune response so that it can protect you against COVID,” says UNC Health family medicine physician Sarah Ruff, MD.
How do you know if a swollen lymph node might be cancerous?
Breast cancer also can cause a swollen lymph node in the armpit. That’s because when breast cancer spreads outside of the breasts, it often goes to the lymph nodes under the arms.
Both Drs. Ruff and Brimmage say that if you feel swelling in your lymph nodes and recently had a COVID-19 vaccine, wait a week to 10 days to see if the swelling decreases. If it does not, go see your doctor. (And know that conditions other than cancer can cause a persistent swollen lymph node, but you’ll want to see a doctor to know for sure.)
“If it doesn’t seem like it’s getting any smaller over the course of about a week, then I think that is definitely time to come in and get it checked out in the office,” Dr. Brimmage says. “If it seems like it’s going down and going away over the course of the week, which it almost always does, then I don’t think any further kind of follow-up or concern is needed—particularly if it’s in the armpit rather than in the main part of the breast.”
Should you delay your mammogram?
Some women are wondering what to do about mammograms: Will an armpit lump from the vaccine complicate the results?
Probably not, Dr. Brimmage says.
“Even if you feel a lump or a bump there in your armpit, it is unlikely that that’s going to be mistaken for a breast lump or a breast nodule that would then lead to biopsy or further imaging,” Dr. Brimmage says. “Worst case, if they weren’t sure on that mammogram, they would either bring you back for a repeat mammogram in a couple of months or they would do an ultrasound of the area.”
If you have a mammogram scheduled and were recently vaccinated but don’t feel any kind of a lump or bump, do not reschedule your mammogram. And even if you do feel something, you don’t have to put off your annual screening—especially if it would be hard to reschedule.
If you don’t have a screening mammogram scheduled but are due for one, the Society of Breast Imaging recommends scheduling screening exams either before your first dose of a COVID-19 vaccination or four to six weeks after the second dose for those COVID-19 vaccines that require two doses.
“You can wait to schedule your mammogram just to avoid any confusion,” Dr. Ruff says.
As always, if you’re unsure what to do, ask your doctor.
“If you find a swelling anywhere, a lymph node in your neck, in your armpit, anywhere, tell your doctor,” Dr. Ruff says. “Your doctor can help you decide if it’s most likely due to the vaccine or if it’s something that they should see you for, and then you can talk about a plan together.”
If you’re worried about swollen lymph nodes or have questions about the COVID-19 vaccines, talk to your doctor or find one near you.