Covid vaccine side effects

Today the FDA released Pfizer’s Phase III vaccine trial data. Pfizer’s report was 92 pages long and the FDA report was 53 pages long. Here are some cliff notes…

Things are looking good. Well, not just good, they are looking great. This trial was as clean as it can get. There is nothing (big) we didn’t expect.


• We see benefit starting 14 days after the first dose (see Figure). This makes sense because it takes about 2 weeks for the immune system to make sufficient antibody protection after vaccination.

• After the first dose, efficacy was 52%. This is a fantastic for a single dose.

• After your second dose (3 weeks later), efficacy is 95%. We are confident that efficacy ranges from 90-98%.

• Efficacy doesn’t meaningfully differ by age, race/ethnicity, or comorbidity.

Adverse events.

• Overall, the Pfizer vaccination gives more adverse events than the flu vaccination but less than the shingles vaccine.

• The most common solicited adverse reaction (i.e. what we were expecting) were mild to moderate: injection site reactions (84.1%), fatigue (62.9%), headache (55.1%), muscle pain (38.3%), chills (31.9%), joint pain (23.6%), fever (14.2%)

• The chance of getting a severe adverse event was low (0.5%). A severe event is more likely in the second dose than the first and were less frequent among older adults compared to younger participants.

• An unsolicited (i.e. not expected) adverse event that came up more in the vaccine group was swollen lymph nodes (lymphadenopathy). 64 people in the vaccine group compared to 6 people in the placebo group. This could be related to the vaccine.

• There were 4 people in the vaccine group that got Bell’s palsy (compared to 0 in the placebo group). These cases occurred at 3,

9, 37, and 48 days after vaccination. This isn’t much of a concern because it reflects the same rate of Bell’s palsy in the general population. Something to keep an eye on

• 6 people died during the trial (4 in placebo; 2 in vaccine group). Deaths were NOT related to the vaccine. Mostly related to heart attacks.

There are a few unknowns. From this study, we do not know…

• Duration of protection

• Effectiveness against transmission

• Effectiveness or safety among certain high-risk populations, such as children less than 16 years of age, pregnant and lactating individuals, and immunocompromised individuals. There were just not enough people in the study to make meaningful conclusions. This likely means that the FDA will not approve emergency use for these populations. We will know more on Thursday.

• Benefits of individuals with prior COVID19 infection. But, given there is documented reinfection, the FDA stated that previously infected people could benefit from vaccination.

Translation: This vaccine will likely be approved on Thursday. Which, by the way, will be streaming live. An event that will likely go down in history, albeit a bit boring, scientific, and long.

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