Do my COVID-19 booster side effects tell me anything about how much immunity I have from the shot?

Bivalent boosters are here and it's *really* exciting because these vaccines contain BA4/BA5, so they are a perfect match to the circulating virus. This gives us a great chance to quell down a winter surge...if enough people get boosted. I got my bivalent booster two weeks ago, and I was EXHAUSTED afterwards. I slept for like 24 hours off and on, and I also had a sore arm and a headache.

Since many of us are getting our boosters and comparing our experiences, I have been asked several times this week why some people have big reactions to the vaccine, and why some people do not. Are people that have big reactions more "immune" to COVID?

Since so much of the vaccine reaction discussion is anecdotal, the best way to approach this question is by looking at clinical trial data for the mRNA vaccines because they are big, well controlled studies, and then we can compare those studies to real world data.

There are two types of vaccine reactions: local and systemic. Local reactions are at the site of the shot like pain, swelling, redness, swollen glands. Systemic reactions are things like fatigue, fever, headache, chills, nausea.

The most common local event by far for both vaccines was pain at the injection site and ~20% of Moderna (Graph 1) & Pfizer (Graph 2) vaccine recipients had none (and it was less common in patients over 55). The other local events were not common.


For the systemic events, the most common reactions were fatigue, headache and myalgia (pain). Moderna had about 45% with no systemic reactions after dose 1 and 20% without a reaction atfter dose 2. Pfizer had higher rates of reactions in their younger 16-54 subgroup than 55+, but reactions were stronger after dose 2, consistent with Moderna.


Comparing these trials to the real world

data from the V-safe system, it’s extremely

consistent (Table 1).

So the takeaway is: The clinical trials measured both local and systemic reactions AND immunity. There were no differences with immunity to the virus, whether a person had vaccine reactions or not. Everyone’s immune system is unique. Some people who are over 55 may [TABLE 1: V-SAFE REAL WORLD DATA]

have a gradual age-related decline in immune

activity, and some people may also be on immunosuppressive medication that causes them to not react strongly.

TL/DR - Vaccine side effects should not be taken as a proxy for effectiveness of someone’s vaccination.