An Epidemiologist on What to Expect From a Covid-19 Vaccine

Pfizer says they have a 90% effective vaccine. What does that mean?

Gideon M-K; Health Nerd
Medium Coronavirus Blog
8 min readNov 10, 2020


Life-saving. Source: SELF Magazine

The holy grail of Covid-19 has been, for most of the year, a vaccine. We’ve all been using “when we’ve got a vaccine” as a synonym for “back to normal,” because the only return to normalcy that we can really envision is based on everyone being immunized against infection. In all the conversations about how the pandemic might end, the one thing that is always featured is a vaccine that prevents the disease in the people who get it.

Which is why the recent Pfizer news feels momentous. The preliminary results from one of the first Phase 3 trials for a vaccine against Covid-19 have been reported, with Pfizer and BioNTech announcing that their vaccine is 90% effective at preventing Covid-19 at 28 days after the vaccine was given. All of a sudden, the hazy dreams of tomorrow where we are all safe from Covid-19 have become much more realistic in the near future.

No longer a distant dream. Source: SELF Magazine

But before we declare the pandemic officially over, it’s worth noting that this is just one more step in the long haul. Yes, these results are promising — and, I personally think, incredibly exciting — but there is a long way to go before we are totally over Covid-19, if that’s possible at all.

So let’s have a look at what has been announced, and what it really means.

The study

The study itself is looking at BNT162b2, an mRNA vaccine candidate developed by Pfizer and BioNTech. It is what’s known as a Phase 3 clinical trial, which means that the companies are testing whether the vaccine actually works to prevent disease in a very large group of people — over 40,000 — using a randomized, controlled study design. This sort of study is the gold standard for finding out whether a vaccine works, and what if any side effects we need to be worried about.

The study design is pretty simple, at heart: One group gets the vaccine, another a placebo, with a second dose at the 21-day mark, and people are followed up at 28 days to see if they have been…