Was Pfizer trial big win for masking?

Sharply reduced infection rate in placebo arm, too

Buried in the scant data released by Pfizer that show heartening early returns on its COVID-19 vaccine: it looks like masking and social distancing work wonders, too.

The topline for the Pfizer vaccine was that it reduced infections by 90%. Of the 38,955 volunteers who received either two doses of the vaccine or placebo between July 27 and November 8, nine or less people in the vaccine arm contracted COVID-19. The other 85 cases or more came in the placebo arm.

If we reduce those numbers to infection rates (there was a little under 19,500 people in each arm of the trial), that boils down to an infection rate of 0.04% among those getting the vaccine compared to 0.4% among those who got nothing but saline solution.

So why might this also be a big win for public health measures to control the disease? Over the course of the same 3 ½ months — when the pandemic was gathering steam due to the politicization of public health here and a worldwide second wave — the U.S. infection rate hit 1.7%. The raw numbers, according to the CDC infection tracker: 5.6 million cases in a population of approximately 325 million people.

In other words, the infection rate in the placebo arm of the trial was less than 25% of the general population: 0.4% compared to 1.7%.

What might account for this discrepancy?

The simplest hypothesis is that people who volunteer for trials are more likely than people in the general population to adopt the prescribed measures for preventing exposure: wearing masks, social distancing, and avoiding group indoor gatherings. It’s possible they were instructed to observe those protocols during the trial.

I don’t know if the trial organizers instructed people on how to behave after they were enrolled. The trial protocols listed on the federal government’s website for tracking trials makes no mention of trial participants getting “usual care,” which in this case would mean having the physicians or nurses administering the vaccine (or placebo) instruct their patients to observe recommended CDC guidelines for preventing the disease, which includes masking and social distancing.

A Pfizer spokeswoman has not yet responded to my email inquiry about that aspect of the trial protocols.

There may be other confounding variables that contributed to the lower infection rate among trial participants. The limited data in Pfizer’s press release doesn’t reveal to what extent there may be under- or over-sampling of different age groups or vulnerable populations.

Just like the vaccine’s ultimate safety and efficacy, we’ll have to wait until all the results are in and the Food and Drug Administration releases the data to get answers to those questions. But based on the early returns, it looks like public health measures like masking and social distancing reduces the infection rate by as much as 75%.

It also suggests that in order to get the 90% efficacy attributed to the vaccine, people will still have to observe public health measures until the virus is largely eliminated from the population.