What You Should Know About Pfizer’s Covid-19 Vaccine Results

Reports of 90% efficacy are really good news, but many questions remain

Photo: NurPhoto/Getty Images

This morning, Pfizer and BioNTech released preliminary data about the efficacy of its Covid-19 vaccine. The news, which has been anticipated almost as fervently as election results, is positive: Based on early analyses, the vaccine is more than 90% effective at preventing Covid-19.

This is really good news. But before you rip off your mask and start hugging people in the street, there are a number of caveats to keep in mind.

The clinical trial — run by Pfizer using a vaccine developed by BioNTech — is being conducted in nearly 44,000 people around the world. The results reported are from 94 cases of Covid-19 that have emerged among those trial participants. Presumably, the vast majority of those cases were in people who received the placebo, but details about the results are scant because Pfizer issued the preliminary findings via a press release, not an academic paper. It’s unclear how many people who received the vaccine still contracted Covid-19, and if so, why.

Similarly, whether the vaccine protected against asymptomatic infections is also unknown; the press release only mentions symptomatic cases of Covid-19. This is an important issue because people who are asymptomatic can still spread the infection.

Another question is whether the vaccine protects against severe Covid-19. Vaccine efficacy for severe versus mild cases was a major point of debate at the meeting of an independent vaccine advisory committee last month. Based on the Food and Drug Administration’s (FDA) current guidelines for authorizing a vaccine, some experts were concerned that a vaccine would only have to prevent mild cases of the disease but not necessarily make an impact on hospitalizations or deaths. The news today does not distinguish between mild and severe cases.

Also missing is specific safety information on adverse events resulting from the vaccine. The press release states, “No serious safety concerns have been observed; Safety and additional efficacy data continue to be collected.” In order to meet the FDA’s authorization criteria, clinical trial participants who have received the vaccine must be followed for a median of two months to monitor for possible side effects. Pfizer says it expects to meet these requirements by the third week of November and will submit for emergency use authorization when it does.

Based on these lingering questions, scientists on Twitter have called for more data to be released and tried to temper expectations for the vaccine. But despite this, enthusiasm is high.

For one thing, the 90% efficacy reported is way beyond the 50% efficacy requirement the FDA had in place. Even if the vaccine ends up being less effective than initially thought after more data are analyzed, or if it’s less effective for either severe or asymptomatic cases, it’s unlikely the overall efficacy will change dramatically enough to prevent it from being authorized.

The results, which are the first to be released from one of the large phase 3 clinical trials currently underway, also bode well for other vaccines in development. Pfizer’s vaccine employs a novel approach that uses messenger RNA to deliver a coronavirus protein into the body. The presence of the protein kicks off an immune response, resulting in the production of virus-specific antibodies that are ready to fight off an infection should the real coronavirus try to enter the body. This type of technology has not been approved for use in vaccines before, but early studies of the new vaccine showed promising results, with high levels of antibodies produced in response to the drug. Moderna’s vaccine uses a similar RNA-delivery system and produced a comparable antibody response, so good safety and efficacy data for one of the RNA vaccine candidates suggests the other might also be effective.

Given the massive manufacturing and logistical hurdles to rolling out a new drug to the world’s more than 7 billion people, experts have long said that several different vaccines may be necessary. Pfizer says that it expects “to produce globally up to 50 million vaccine doses in 2020 and up to 1.3 billion doses in 2021.”



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Dana G Smith

Dana G Smith


Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental