“Which arm?” That is a common question when you get your first and second COVID 19 jabs. We usually

take for granted that it doesn’t matter. Which arm to receive the 1 st and the 2 nd doses is totally a decision

from personal habit and it will make no differences on the vaccination results. However, a group of

prudent scientists from Germany showed that to receive the 2 nd jab on the same arm as your 1 st one or

on the different arm made big differences on the immune responses! The title of the research paper is

“Differences in SARS-CoV-2 specific humoral and cellular immune responses after contralateral and

ipsilateral COVID-19 vaccination”. It was published on eBioMedicine, August 11, 2023

(DOI:https://doi.org/10.1016/j.ebiom.2023.104743).

In this research, 303 individuals (gender and age are evenly mixed), who have never received COVID-19

vaccines or have never been infected with COVID-19, were divided into two groups. Group one received

both shots of the Pfizer vaccines on the same arm and group two on different arms. Blood samples were

tested 14~15 days after the 2 nd vaccination. Strong immune responses were induced by vaccinations in

both groups. Similar spike-specific antibody levels and antibody binding activities were observed in both

groups; however, humoral immunity from group two (jab on different arms) were much lower than

group one (jab on the same arm), as indicated by the antibody neutralizing abilities. Cellular immunity

activity from group two was also significantly weaker than group one, because spike-specific CD8 T-cells,

together with the percentage of individuals with detectable CD8 T-cells, were both much lower in group

two than group one.

What is the reason that caused such differences in the immune responses? The researchers speculated

that vaccination on the same arm forced the vaccines to go through the same lymph drainage and

strongly stimulate the same lymph nodes. On the contrary, boosting on a different arm reduced the

stimulation levels to the activated lymph nodes, because the lymph circulation on the left and right side

of the body is separated.

Based on these research results, probably the next time when you receive your boosting jab, you should

expose the same biceps as you received the first one.

Hope you still remember which arm the first jab was on!