But fewer cases of Covid-19 occurred among trial participants in the early days of the Moderna trial, making it harder to measure differences between the vaccinated group and the placebo group. In any case, health officials have said that for both vaccines, two doses are essential for full protection.
A second difference involves the ability to prevent severe disease. Moderna produced more evidence that its vaccine can do so, according to the review. In its trial, 30 volunteers developed severe cases of Covid. All of them were in the placebo group, with no cases among the people who were vaccinated.
In the Pfizer-BioNTech trial, the findings were less compelling. There were 10 severe cases in the placebo group and one in the vaccinated group. Those numbers are too small to gauge the vaccine’s ability to prevent serious illness.
“The available data for these outcomes did not allow for firm conclusions,” the F.D.A. said.
The documents published Tuesday made clear that side effects were particularly common after the second dose, but typically lasted only a day. Experts say people may need to take a day off work after getting the shot.
During the Moderna trial, researchers also kept an eye out for volunteers who developed new disorders. In a trial lasting several months with 30,000 volunteers, it is normal for some to come down with conditions that have nothing to do with the vaccine, health experts say. Comparing the rates between the people who get the vaccine and the placebo — as well as general background rates — can help pinpoint serious concerns and rule out coincidences.
Confused by the terms used about how to treat Covid-19? Let us help:
- ACE-2: A protein that sits on the surface of certain types of human cells. The coronavirus has to bind to ACE-2 in order to enter cells.
- 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.
- Antiviral drug: A drug that interferes with a virus’s ability to replicate inside cells. The first approved drug for Covid-19 in the United States, remdesivir, is an antiviral.
- 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.
- Compassionate use: A term used to describe treatments that are given to seriously ill people despite not yet being approved by the Food and Drug Administration for that use.
- Cytokine storm: A hyperactive response from the immune system that can lead to massive amounts of inflammation and tissue damage. Cytokine storms may be responsible for many of the severe cases of Covid-19, and a number of researchers are testing drugs that might be able to quiet them.
- Interferon: A molecule made by the immune system. Certain types of interferons can unleash inflammation in the body, while others tamp it down. Yet other types can spur cells to strengthen their defenses against viruses. Researchers are exploring whether treatments of synthetic interferons can help people fight off the coronavirus.
- Monoclonal antibodies: Monoclonal antibodies, created in a laboratory, mimic the natural antibodies produced by the immune system. A number of companies have developed these treatments for Covid-19. President Trump received Regeneron’s antibody treatment shortly after he was diagnosed with the disease.
- 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 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.
- Trial protocol: A series of procedures to be carried out during a clinical trial.
- Retrospective study: A study that analyzes data collected in the past to assess how effective a treatment is. Retrospective studies can offer useful insights, but are not as definitive as randomized clinical trials.
- Spike protein: A protein that sits on the surface of coronaviruses. The spike protein binds to the ACE-2 receptor on human cells using a region called the receptor-binding domain (R.B.D.). Once the protein attaches, the virus can enter the cell. Many vaccines and monoclonal antibody treatments are designed to attach to the spike.
- Standard of care: A treatment accepted by medical experts as a proper way to treat a certain type of disease. Once a standard of care emerges for a disease, any new experimental treatments are typically tested against it, rather than against a placebo.
During the Moderna trial, three vaccinated participants developed a form of temporary facial paralysis called Bell’s palsy, while one participant with the placebo also experienced it. Bell’s palsy, which can last weeks or longer, can be triggered by viral infections and other causes. Some 40,000 people develop the condition each year in the United States. Years of intense research has failed to find evidence that any vaccine routinely recommended in the United States causes Bell’s palsy.
In the review released on Tuesday, the F.D.A. said: “Currently available information is insufficient to determine a causal relationship with the vaccine.”