By now, most people are familiar with the typical way of administering the Covid-19 vaccine: a quick bump on the upper arm. However, there is one lesser-known part of the body where the vaccine has been approved for injection: the thigh.

While getting the vaccine in the thigh is rare, there are some groups of people who might want to consider it. If you fall into any of the categories below and think that it is better to get the Covid-19 vaccine in your thigh than in your arm, this should be discussed with your doctor first.

Lymphedema is a chronic and painful condition that causes swelling in parts of the body. For example, it can develop in breast cancer patients who need surgery to remove lymph nodes from under the arm. Removal of the lymph nodes disrupts lymph flow, the extra fluid from tissues that would normally drain through the lymphatic system into the bloodstream, causing the fluid to retreat and swelling in the chest, trunk, or arm on the affected side.

In both Moderna and Pfizer-BioNTech clinical vaccine studies, some participants had swollen lymph nodes in their armpit or neck area two to four days after vaccination on the same side as the shot in the arm. This is a normal short-term side effect that means the body is responding to the vaccine. In the case of Moderna, the mean duration of the swelling was one to two days and averaged 10 days in those who received Pfizer-BioNTech.

However, this side effect may be important in patients with lymphedema or at risk of lymphedema. If someone has lymphedema in both arms, or if a patient is at risk of lymphedema in both arms, some medical institutions recommend their patients to receive the Covid-19 vaccine in the thigh as a precautionary measure. The concern is that the vaccine could either make the arms swell even more or, for those at risk of lymphedema, cause worrying symptoms where they weren’t.

The immune response can also be less efficient if the shot is given in an arm with no lymph nodes or in an arm with compromised lymphatic drainage.

“The lymphatic vessels are responsible for the drainage of fluid from all of our tissues. If your lymphatic drainage is not good, your tissues will swell out of the fluid – and that would also mean that you would not be transporting a vaccine from tissue to lymph node very efficiently, ”said Marc Jenkins, director of the Center for Immunology at the medical school from the University of Minnesota.

Patients with lymph nodes removed from one arm may be more likely to receive the vaccine in the unaffected arm than in the thigh, said Cheryl Brunelle, associate director of the lymphedema research program at Massachusetts General Hospital in Boston.

Swollen lymph nodes in the armpit can be a sign of breast cancer. If someone with a history of breast cancer did not know that the Pfizer BioNTech or Moderna vaccines could produce swollen lymph nodes, it could be “very scary to them and think it might be a recurrence,” Ms. Brunelle said.

To address potential concerns, people with a history of breast cancer can insert the vaccine into the thigh if they wish.

Coronavirus vaccinations can cause enlarged lymph nodes in the armpit or near the collarbone that appear as white spots on mammograms and may be mistaken for signs of cancer. The Society of Breast Imaging recommends that you try to schedule your routine screening mammogram before your first dose of Covid-19 vaccine or at least a month after your second dose of vaccine. However, an alternative plan would be to get the vaccine in your thigh instead.

Updated

April 29, 2021, 10:13 p.m. ET

“It is extremely unlikely that an injection in the thigh would cause swelling of the armpit nodes,” said Dr. Constance D. Lehman, head of breast imaging at Massachusetts General Hospital, told the New York Times earlier this month.

If you’d rather hold the vaccine in your arms and have already scheduled your mammogram, you can keep that appointment – as well as your vaccine appointment – and call your breast imaging center in advance to let them know when you will have your vaccine.

If you have had the Covid vaccine in your arm in the past six weeks, your radiologist will expect the lymph nodes to swell on the same side that you received the vaccine. This would be a normal finding unless the swelling lasted more than six weeks or there were other clinical concerns. In that case, they would take more pictures as needed, Ms. Brunelle said.

This might take some perseverance. Covid-19 vaccination centers primarily offer arm vaccinations: it’s quick, efficient, and there is no need to find a private room to undress. Because the vast majority of people receive arm injections, some vaccination centers may not have staff trained to give it in the thigh.

“I only advise patients to call ahead, inform the vaccination clinic or pharmacy that they are asking about the vaccine in the thigh,” said Ms. Brunelle. “I also advise patients that the patient can approve the CDC Rules of Procedure document listing the thigh as an alternative location for Pfizer and Moderna if a local facility or vaccine-giving doctor does not approve of the thigh alternate location familiar is vaccinations. “

In New York City, people visiting health department vaccination sites can speak to staff if they have concerns about getting the vaccine in their arms, said Michael Lanza, a health department spokesman. These locations also have “quiet rooms” where thigh injections can be done discreetly, he added, but “this should only be done if medically necessary.”

Experts say you shouldn’t worry if you’ve already been vaccinated on the side that you had breast cancer on or if you were vaccinated in an arm that had fewer lymph nodes. The important thing is that you received the vaccine.

“We don’t want to scare women who have already been vaccinated,” said Dr. Alphonse Taghian, director of the lymphedema research program at Massachusetts General Hospital. “It is possible that there will be no problem at all.”

To learn more, Dr. Taghian and Ms. Brunelle asked current and former breast cancer patients about the side effects of the vaccine to better understand how many patients report swollen lymph nodes and when and for how long the symptom occurs. They hope to see results before the end of the year.

Dr. Taghian encourages patients with a history of cancer to contact their oncology team if they have any questions or concerns, and to let their doctor know if they develop any new signs of swelling.

The Covid-19 vaccines are designed to be injected into muscles. The muscles of the upper arm are good for shooting and are considered less painful than some other areas of the body. However, a vaccine can be injected into other muscles, provided they are near some of the hundreds of lymph nodes that are present in the body. For example, the thigh is located near several lymph nodes and has been shown to produce an effective immune response after vaccination.

Lymph nodes are “absolutely critical to generating immune responses,” said Dr. Akiko Iwasaki, professor of immunobiology at Yale University.

When a vaccine gets into the muscle of the arm or thigh, it is transported to a nearby lymph node, she added. There, the vaccine is absorbed by special cells that teach white blood cells, called T and B cells, to either become killer cells that seek and destroy cells infected with coronavirus, or antibody-secreting cells.

Decades ago, other vaccines were often injected into the buttocks. Scientists now know that the layers of fat cells in our soil are too numerous to allow easy access to muscles and lymph, making the immune response inefficient. In addition, vaccination is generally not given to the buttocks to avoid injury to the sciatic nerve.