How the different vaccines work
How do these new genetic vaccines work?

The Pfizer-BioNTech and Moderna vaccines use a genetic molecule to prime the immune system. That molecule, known as mRNA, is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. While the immune protection from these vaccines may last for months or perhaps even years, their mRNA does not — it is destroyed by our cells within days. Although these are described as “genetic” vaccines, the vaccines don’t alter your genes in any way. —Carl Zimmer
What do the vaccine developers mean when they say their vaccines are 95 percent effective?

Vaccine developers test their vaccines in clinical trials. The fundamental logic behind these trials was worked out by statisticians over a century ago. Researchers vaccinate some people and give a placebo to others. They then wait for participants to get sick and look at how many of the illnesses came from each group.

In the case of Pfizer, for example, the company recruited 43,661 volunteers and waited for 170 people to come down with symptoms of Covid-19 and then get a positive test. Among those who got sick, 162 had received a placebo shot, and just eight had received the real vaccine. This result shows that receiving a vaccine dramatically lowered the chances of getting Covid-19 compared to receiving a placebo.

The difference is expressed as efficacy: the Pfizer-BioNTech vaccine has an efficacy rate of 95 percent. (If there were no difference between the vaccine and placebo groups, the efficacy would be zero. If none of the sick people had been vaccinated, the efficacy rate would have been 100 percent.) A vaccine’s efficacy rate and effectiveness rate are different: Efficacy is a measurement made within the strict confines of a clinical trial, whereas effectiveness refers to how a vaccine works in the real world. No one knows yet the true effectiveness of these new vaccines. —Carl Zimmer
Was the Pfizer vaccine part of the government’s Operation Warp Speed?

Pfizer did not accept federal funding to help develop or manufacture the vaccine, unlike front-runners Moderna and AstraZeneca. Pfizer did get a $1.95 billion deal with the government to deliver 100 million doses of the vaccine. The arrangement is an advance-purchase agreement, meaning that the company won’t get paid until they deliver the vaccines. Read more here. —Carl Zimmer and Katie Thomas
What does the rollout of the Pfizer vaccine mean for the other vaccines in the race?

Researchers were heartened by the strong results of the vaccine developed by Pfizer and BioNTech. “It gives us more hope that other vaccines are going to be effective too,” said Akiko Iwasaki of Yale University.

The Moderna vaccine, which is next in line for approval, has an efficacy rate of 94.5 percent, essentially the same as the Pfizer-BioNTech vaccine. A vaccine from AstraZeneca and the University of Oxford has shown confusing trial results, with efficacy rates between 60 and 90 percent, depending on the strength of the doses that volunteers received. And the French company Sanofi had a major disappointment in its early clinical trials, finding that its vaccine couldn’t provoke an immune response in people over 55. The company is now reformulating its vaccine to start new trials. —Carl Zimmer and Katie Thomas

Reporting contributed by:

Abby Goodnough, Carl Zimmer, Rebecca Robbins, Apoorva Mandavilli, Denise Grady Katie Thomas, Tara Parker-Pope, Noah Weiland, Natasha Singer, David Leonhardt, Roni Caryn Rabin, Julie Bosman, Reed Abelson and Richard Pérez-Peña The New York Times