What’s up with varying vaccine side effects?


Q: I took my second Pfizer vaccine along with several friends and my mom. We all had minimal to no side effects. Is that worrisome that it didn’t “take”? Do side effects frequency vary with age or by vaccine?

A: Good questions! So glad you got your shot!

While “reactogenicity” symptoms are common, not having any side effects from the vaccine does not mean that the vaccine wasn’t effective. For the folks that do have side effects, most are mild and go away after 1-2 days. Younger people, people who have had Covid19 before, and people who got the Moderna vaccine are more likely to experience side effects.

The #Covid19 vaccines are often “reactogenic.” This means that many people experience physical manifestations of the immune system kicking in and ramping up, such as pain at the injection site, fatigue, headache, muscle aches, and fever. But if you don’t have these symptoms, the vaccine is still working!

Ok, let’s look at the numbers. For the #Pfizer-BioNTech vaccine, 88.7% of people aged 18 to 55 and 79.7% of people older than 55 reported at least one local reaction (most commonly pain at the injection site). Of people experiencing pain, a little over half only had mild symptoms (meaning it did not interfere with their activities). The #Moderna vaccine showed slightly higher rates of local reactions (again, most often pain) with 90.5% of people aged 18 to 64 reporting a local reaction and 83.9% of people aged 65 and up. The majority of those were mild as well. Both vaccines had more reactions in younger people and were more common after the second dose.

Systemic reactions are also common but not a given. Systemic symptoms include things like fever, headache, body aches, and feeling really tired. In the Pfizer-BioNTech trial, 77.4% of participants reported at least one systemic symptom. After the second dose, 81.9% of the younger group in the Moderna trial (age 18-64) reported a systemic reaction compared to 71.9% of the older group (age over 65). Once again, both vaccines had more reactions in younger people and were more common after the second dose. Severe side effects for both were rare, but slightly more common in the Moderna vaccine trial. For example, people receiving the Moderna vaccine reported severe fatigue about 10% of the time after the second dose as compared to about 5% of the time for Pfizer recipients. Moderna vaccine recipients were more likely to report severe headache, joint pain, and myalgias than Pfizer vaccine recipients.

Major point to remember: severe symptoms were rare for BOTH vaccines, there were NO deaths in either trial due to the vaccine, and both are VERY effective.

So, while many people did have a vaccine side effect, most people had mild or sometimes no side effects at all. The immune system still works, even when it isn’t showing off by giving you symptoms. Both vaccines are about 95% effective at preventing symptomatic Covid19. Both are great at preventing hospitalization and death. Regardless of whether or not you have any vaccine side effects.

More good news: the #CDC recommends against checking for antibodies after the vaccine to see if you developed an immune response. One less needle poke is always appreciated.

The super handy charts included here are from Goodrx.com. We Nerdy Girls love a good chart.


CDC Summary of Pfizer-BioNTech Reactogenicity

CDC Summary of Moderna Reactogenicity

GoodRx Summary of Vaccine Side Effects

Link to original FB post