The companies said that out of 170 cases of Covid-19, 162 were in the placebo group, and eight were in the vaccine group. Out of 10 cases of severe Covid-19, nine had received a placebo.
Pfizer and BioNTech said that the vaccine’s efficacy was consistent across age, race and ethnicity. The most common serious adverse event was fatigue, with 3.7 percent of volunteers reporting tiredness after they took the second dose. Two percent of volunteers reported a headache after the second dose. Older adults reported fewer and milder side effects, the companies said.
“It’s pretty amazing,” said Akiko Iwasaki, an immunologist at Yale University. She said the results in people over 65 were the most promising. “We know from the influenza vaccine that it’s very difficult to achieve protection in this age group with vaccines,” she said, so 94 percent efficacy in that group “is really remarkable.”
Federal health officials have said the first doses of the vaccines will most likely go to groups like health care workers who are at high risk for exposure, as well as to people who are most vulnerable to the disease, such as older people.
Confused by the all technical terms used to describe how vaccines work and are investigated? Let us help:
- Adverse event: A health problem that crops up in volunteers in a clinical trial of a vaccine or a drug. An adverse event isn’t always caused by the treatment tested in the trial.
- Antibody: A protein produced by the immune system that can attach to a pathogen such as the coronavirus and stop it from infecting cells.
- Approval, licensure and emergency use authorization: Drugs, vaccines and medical devices cannot be sold in the United States without gaining approval from the Food and Drug Administration, also known as licensure. After a company submits the results of clinical trials to the F.D.A. for consideration, the agency decides whether the product is safe and effective, a process that generally takes many months. If the country is facing an emergency — like a pandemic — a company may apply instead for an emergency use authorization, which can be granted considerably faster.
- Background rate: How often a health problem, known as an adverse event, arises in the general population. To determine if a vaccine or a drug is safe, researchers compare the rate of adverse events in a trial to the background rate.
- Efficacy: A measurement of how effective a treatment was in a clinical trial. To test a coronavirus vaccine, for instance, researchers compare how many people in the vaccinated and placebo groups get Covid-19. The real-world effectiveness of a vaccine may turn out to be different from its efficacy in a trial.
- Phase 1, 2, and 3 trials: Clinical trials typically take place in three stages. Phase 1 trials usually involve a few dozen people and are designed to observe whether a vaccine or drug is safe. Phase 2 trials, involving hundreds of people, allow researchers to try out different doses and gather more measurements about the vaccine’s effects on the immune system. Phase 3 trials, involving thousands or tens of thousands of volunteers, determine the safety and efficacy of the vaccine or drug by waiting to see how many people are protected from the disease it’s designed to fight.
- Placebo: A substance that has no therapeutic effect, often used in a clinical trial. To see if a vaccine can prevent Covid-19, for example, researchers may inject the vaccine into half of their volunteers, while the other half get a placebo of salt water. They can then compare how many people in each group get infected.
- Post-market surveillance: The monitoring that takes place after a vaccine or drug has been approved and is regularly prescribed by doctors. This surveillance typically confirms that the treatment is safe. On rare occasions, it detects side effects in certain groups of people that were missed during clinical trials.
- Preclinical research: Studies that take place before the start of a clinical trial, typically involving experiments where a treatment is tested on cells or in animals.
- Viral vector vaccines: A type of vaccine that uses a harmless virus to chauffeur immune-system-stimulating ingredients into the human body. Viral vectors are used in several experimental Covid-19 vaccines, including those developed by AstraZeneca and Johnson & Johnson. Both of these companies are using a common cold virus called an adenovirus as their vector. The adenovirus carries coronavirus genes.
- Trial protocol: A series of procedures to be carried out during a clinical trial.
Dr. Saad B. Omer, the director of the Yale Institute for Global Health, said the results showing the vaccines protected people from severe disease was also good news, because with such limited availability initially, the first goal will be not to stop transmission of the disease, but to prevent people from becoming extremely ill. “So that is very reassuring,” he said.
He called on Pfizer to quickly release a more detailed analysis of the data — beyond the initial news release — so that scientists could more thoroughly evaluate the results. The companies have said they plan to submit the results for review in a scientific journal, a process that can take weeks or months. “These are not normal times, this is a pandemic, and major policy is being made,” Dr. Omer said. “It’s always better to make policy on scientific data, not a press release.”
In addition to the results of its clinical trial, Pfizer said it was ready to submit to the F.D.A. two months of safety data that the agency had recommended, as well as detailed manufacturing records showing the company can consistently produce high-quality batches of its product. The F.D.A. will review the data and ask an outside panel of vaccine experts to weigh in on the application, a process that could take weeks.