How to think about vaccine results

This past Monday, pharmaceutical company Pfizer announced preliminary clinical trial results for its COVID-19 vaccine. In an interim analysis of the vaccine’s phase 3 study, the vaccine was shown to be 90% effective in preventing COVID-19. In other words, based on the people in Pfizer’s study who have become diagnosed with COVID-19 so far, those who got vaccinated were 90% less likely to get sick compared with the people who did not.

90% is an exciting number. The Food & Drug Administration (FDA) set a threshold of 50% effectiveness for COVID-19 vaccines to be authorized, and experts have been telling us for months that even a 60% or 70% effectiveness would still be incredibly useful in reducing infections across the population. Pfizer’s initial 90% rate blows those expectations out of the water.

Plus, this effectiveness value bodes well for other vaccine candidates. Pfizer’s vaccine, developed through a partnership with German biotech BioNTech, uses a new vaccine technology based on synthetic messenger RNA, or mRNA; so does the vaccine developed by Moderna, which is also currently in clinical trials. (For more backstory on mRNA, BioNTech, and Moderna, I highly recommend Damian Garde’s feature in STAT News.)

But we can’t get too excited. Pfizer reported its preliminary data not in a peer-reviewed scientific paper, but in a press release, and some key details about the company’s clinical trial are not yet public. I used information from STAT NewsKHN, and SciLine to compile a few key questions that should be in all of our minds as we think about this and future vaccine data releases.

  • What is the sample size? Or, how many people were involved in the trial, and how many of them were diagnosed with COVID-19? For Pfizer’s trial, this is a question we can answer: about 44,000 people are enrolled in the study, and the 90% effectiveness rate is based on results from 94 people who contracted COVID-19, the majority of whom did not receive a vaccine dose. This may seem like a tiny fraction of the participants, but many experts are cautiously optimistic in hoping the 90% rate will hold up for a larger group.
  • Who is included in the sample size? COVID-19 has disproportionately impacted the elderly, people of color, people with certain medical conditions, and other marginalized groups. It is thus crucial that a vaccine is effective for people in these groups—in other words, these people must be represented in the vaccine trial. Pfizer reports that 42% of the overall study participants have “diverse backgrounds,” but the specific backgrounds of the patients who got sick are unknown.
  • Does the vaccine work for severe cases? While the majority of people diagnosed with COVID-19 are able to survive the disease with mild symptoms in their own homes, the minority of people who become seriously ill constitute the pandemic’s massive loss, as well as its burden on our nation’s healthcare system. A vaccine that reduces the disease’s severity through boosting immune system defenses may be incredibly valuable, even if it does not entirely prevent infection.
  • Does the vaccine work for mild or asymptomatic cases? A vaccine that prevents mild cases would help keep COVID-19 spread at bay, even if this vaccine does not reduce the disease’s severity. Pfizer’s press release does not include any specifics on the 94 patients who were diagnosed with COVID-19; experts are hoping that such details may be revealed in a forthcoming scientific paper.
  • Does the vaccine have any adverse effects? In other words, is the vaccine safe? We all know that flu vaccines make our arms sore, and other vaccines can give us mild colds. These types of common effects are usually nothing to worry about, but vaccines may pose a more severe danger to a small fraction of the population; for example, one in every ten thousand patients might have an allergic reaction that sends them to the hospital. So far, Pfizer has not reported any severe effects of its vaccine, but the current clinical trial gives the company a much wider pool of people in which dangerous reactions might be observed.
  • What are the vaccine’s logistical needs? One dose or two? At what temperature does the vaccine need to be stored? How long can it be at room temperature before it needs to be administered? How many doses can be manufactured in a day, a week, a year? What’s the price tag? Pfizer has given preliminary answers to some of these questions (two doses, -70 degrees Celsius) but the company is finalizing its manufacturing and distribution strategies as it completes its clinical trial.

Even when a vaccine is authorized by the FDA, distributing and tracking it poses a whole new set of questions. I’ve written about vaccine data before, and I expect that this will be a topic I cover in increasing detail during the months to come.

Sign up for the COVID Data Dispatch newsletter

Leave a Reply