Does It Matter Which Arm Gets a Vaccine? Maybe, Maybe Not

According to a study by researchers in Germany, side matters when it comes to vaccinations and boosters. Specifically, their evidence suggests that for the COVID-19 vaccine, you should choose the same arm that got your first dose for the booster dose.

But is that right? (or left)?

"Most people get vaccines on the same side every time because they use their non-dominant arm," says immunologist Manar Abdalgani, MBBS. "This new German study tells us the inclination may have benefits beyond protecting the dominant hand."

In this study, the antibodies in people vaccinated in the same arm were more effective than the antibodies in people vaccinated in different arms.

But, another recent study suggested that vaccines were more effective in people vaccinated in different arms.

In other words, says Abdalgani, who specializes in diseases that affect the immune system, there is not yet enough data to recommend one method of vaccination over the other. 

We asked Abdalgani to tell us more about how our bodies fight germs, what role our arms play, and if it's better to use the same one for every shot.

What is immunization?

Immunization is a process that helps the immune system get ready to fight infections—supporting it to make it stronger and resist a germ or disease.

Immunization is usually done through vaccination. Vaccines imitate infections. They mimic germs to trigger the body's natural defenses, activating the immune system to be ready to defend against the real germ. Activated immune cells and antibodies safeguard against disease and prevent infections from spreading.

What is a "booster dose"?

The first dose of a vaccine, called the primary dose, starts developing immunity against a germ or disease.

A booster dose is another dose of the same vaccine given after the primary. The goal of the booster dose is to boost existing immunity acquired from the primary dose to continue protection and produce more antibodies.

Booster doses are sometimes needed for people who are immunocompromised to help promote and develop some protection against diseases caused by these germs.

Why do you get vaccine shots in the upper arm and not another body part?

Vaccines are shot into the deltoid because muscles absorb antigens quickly, and they have a great supply of blood that helps disperse the vaccine. It's the ultimate place for a vaccine to go, even better than directly into the bloodstream, where it's vulnerable to destruction.

It's also easier to inject a vaccine into a muscle and has few side effects, like inflammation. That said, a little pain, swelling, or redness is a good thing because it shows your immune system is doing what it's supposed to do.

Should you use the same arm for vaccines in general and multiple-dose vaccines in particular?

Maybe. There is limited data suggesting a benefit.

A small, recent study of COVID vaccination in adults found:

• Both ipsilateral (same arm) and contralateral (different arm) vaccination induced a strong immune response, and
• The ipsilateral group had antibodies that did a better job blocking germs from entering cells and
• The ipsilateral group had higher levels of the specialized immune cells that kill infected cells, potentially reducing the spread of infections and disease.

This study shows a potential advantage of ipsilateral vaccination. Studies on other vaccines in children have produced conflicting results. There is not enough data to adopt a recommendation.

This said, vaccination on the same arm is generally easily followed and is favored by most patients. There is no harm in using the same arm.

What's the most common question you get from immunocompromised patients?

Is it safe for me to get vaccinated?

People whose immune systems are compromised have different considerations. I always tell them there are two types of vaccines: Killed and live attenuated.

• Killed vaccines, like the Flu and COVID shots, cannot cause disease. They are generally safe for everyone, including people with compromised immune systems. However, some immunocompromised people may not develop a sufficient immune response due to their compromised ability to make antibodies. In such cases, people can still receive passive immunity against these germs through medications like regular antibody infusion therapy, often prescribed to treat their immunodeficiency.
• Live attenuated vaccines are made using a weakened version of the germ. The germ is suppressed so as not to cause disease but to be viable enough to trigger a desired immune response and offer protection in people with healthy immune systems. In people who have moderately or severely compromised immune systems, the weakened germ could become active, bypass the weak immune system, and cause disease.

Every immunocompromised person should talk to their doctor before getting a vaccine.

What do you wish everyone knew about getting vaccines?

Vaccination is essential to protect human health from serious and potentially life-threatening infections. For example, vaccination led to the eradication of smallpox, which, according to WHO, killed 300 to 500 million people in the last century.

It's true adverse events exist, but the majority are mild and go away within a few days. Natural infections are more likely to make you sicker than the effect of the vaccine itself. Discuss this with your physicians if you think you have allergies or are likely to react poorly. 

Vaccines are effective and safe, and there is absolutely no evidence that they contribute in any way to the development of autism. The CDC and FDA take great care to make sure all vaccines are safe before they are licensed for the public.

Even if you're healthy, it is important to get vaccinated to protect yourself from severe illnesses and protect other people, especially those who are more vulnerable to bad illnesses because of other factors such as a compromised immune system.