A question I hear a lot: How much immunity from one dose of COVID vaccines (Pfizer & Moderna) are you getting?

First, it’s important to remember that the trials were done when the COVID variants weren’t making up a big portion of new cases. So take that into consideration as well. The new variants are a significant percentage of new cases, and we don’t know how effective the vaccines are against those strains.

Secondly, it’s important to know how the COVID vaccines trials defined “effectiveness.”

Every trial has a primary outcome that it is “powered” to figure out. How do you power a trial? It’s based

on a pre-determined formula that essentially tells you how many people you need to include in a trial to accurately assess the primary outcome.

In this case, the primary outcome was effectiveness defined as “reduction in symptomatic cases.” Symptomatic cases are people who test positive for COVID and show symptoms. ( An asymptomatic case is someone who has the virus but doesn’t show symptoms.)

Thirdly, it’s important to remember that you are not considered “fully vaccinated” until 2 weeks after your 2nd dose. This is something to keep in mind for businesses, school, holiday get-togethers and venues. I have no idea if they are going to go forward with “vaccine passports” ( I really hope they don’t, for a lot of reasons), but if they do, you will most likely be limited in what you can do unless you are fully vaccinated.

The Pfizer trial involved around 44,000 people, so you can assume half got the vaccine, and half didn’t. The 2-dose vaccine was 95% effective at reducing cases of symptomatic COVID-19, 7 days out from the second dose.

It was 94% effective in people 65 and older.

An additional analysis looked at how effective just 1 shot would be:

They concluded that the Pfizer shot is 52% effective at reducing symptomatic cases, 12 days AFTER the first dose. ( NEJM, 10 Dec, 2020)

**This analysis was done on a small group of people, so take it with a grain of salt. It also used a statistical technique called mITT, which I don’t have a lot of confidence in. It’s a statistical technique that can result in bias in clincial trials.

Approximately 30,000 people were enrolled in the Moderna trial, so you can assume half got the vaccine and half didn’t. The Moderna vaccine was 95% effective at reducing cases of symptomatic COVID-19, 14 days out from the second dose.

It was 87% effective in people 65 and older.

The Moderna vaccine was 80% effective after just 1 dose, with a median time of 28 days out, BUT (and that’s a big BUT) the sample size used to determine this was small and non-random, and the document itself says to not rely on this number. ( FDA VRBPA Briefing Document, Dec 17, 2020, page 28.)

Take-home point: You’ll most likely get some immunity after the 1st dose, but it’s unclear how much. We might get a more accurate idea as time goes on and more data is collected. If you start a 2-dose vaccine process, it’s best to finish the 2-dose series.