Your questions about coronavirus vaccines, answered

The CDC says that if you are fully vaccinated, you can resume all your usual activities without masks or physical distancing in most cases, even when you are indoors or in large groups. However, you still need to follow guidance at your workplace and any rules in effect at any businesses you visit, as well as state and local restrictions, if those are more stringent.

If you travel, you are still required to wear a mask on planes, buses, trains and other forms of public transportation.

You are considered fully vaccinated two weeks after you receive the one-dose Johnson & Johnson shot, or two weeks after the second dose of either the Pfizer-BioNTech or Moderna vaccine.

Even after that point, you should still watch out for symptoms of covid-19, especially if you’ve been around someone who is sick. If you have symptoms, you should get tested and stay home and away from others.

If you have a compromised immune system, or are taking medications such as steroids that weaken the immune system, you should talk to your health-care provider to discuss your activities. You may need to keep taking precautions to prevent covid-19.

You do not need to quarantine or be tested if you are exposed to the virus as long as you do not develop symptoms. Fully vaccinated employees of high-density workplaces, such as meatpacking plants, who do not have symptoms also do not need to quarantine after an exposure. But a test is recommended to be certain.

Fully vaccinated international travelers coming to the United States are still required to get tested within three days of their arrival or show documentation that they’ve recovered from covid-19 in the past three months. They should still get tested three to five days after their trip.

The CDC updated its previously more cautious guidance on May 13, citing falling infection rates in the United States and real-world evidence of the effectiveness of the coronavirus vaccines even against more contagious variants circulating in the country. Officials also noted the rarity of breakthrough infections in those who are fully vaccinated and the lesser severity of the relatively few infections that have occurred.

Coronavirus vaccines may be less effective in some people with weakened immune systems, including people on heavy-duty cancer drugs, for example.

Laura Makaroff, a senior vice president for the American Cancer Society, said immunocompromised people should continue masking in public settings until they can consult their doctors about the precautions they should take.

Some pediatricians are suggesting that parents of children under 12, who cannot get vaccinated yet, continue to follow guidance from earlier in the pandemic. Grace Lee, associate chief medical officer for practice innovation at Stanford Children’s Health, said gatherings of children should be small and outdoors.

“If I had kids under 12, I would want them to wear a mask in school,” she said. “Wear a mask when we’re in a public space with a lot of people. And indoors, for sure I would have the mask.”

All the vaccines authorized by the Food and Drug Administration — Pfizer-BioNTech, Moderna and Johnson & Johnson — passed rigorous safety reviews. Some experts expressed qualms about using emergency-use authorizations for coronavirus vaccines that would be given to hundreds of millions of people, but those criticisms have become muted as the pandemic has raged, killing thousands of Americans a week.

Peter Marks, director of the FDA center that oversees vaccines, pledged to use an emergency standard roughly equivalent to what is needed for a full licensure. Even so, the available safety data — two months of follow-up on half the trials’ participants after their second shots — is shorter than in traditional trials. And some questions, such as the duration of protection, cannot be answered now. That data will be collected as the trials continue.

Before authorizing the Pfizer-BioNTech vaccine for people ages 16 and over, the FDA reviewed data from 44,000 participants in a randomized clinical trial. A 53-page analysis by the agency found that some people who received injections had unpleasant but tolerable side effects, including fatigue, headaches, muscle pain, joint pain, chills and fever.

For adolescents, the most common side effects of the Pfizer-BioNTech vaccine were similar to those seen in adults but occurred somewhat more frequently. For example, 63 percent of participants 16 and older reported fatigue, but 78 percent of those 12 to 15 years old did. The vaccine also has caused very rare anaphylactic reactions.

The FDA cleared the Moderna vaccine for people ages 18 and over, based on data from a clinical trial with 30,000 participants. The agency found that its side effects were similar to those with the Pfizer-BioNTech shot. For both Pfizer-BioNTech and Moderna, side effects were most common after the second dose.

The Johnson & Johnson vaccine was authorized for people ages 18 and over, based on data from a trial with 44,000 participants. Side effects were less common than for Moderna and Pfizer-BioNTech and included pain at the injection site, fatigue and headaches.

On April 13, federal health officials called for a pause in the use of the Johnson & Johnson vaccine to review reports of six U.S. cases of a rare and severe type of blood clot among the more than 7 million people who had received the shot. The pause was lifted 10 days later as officials said the vaccine’s benefits far outweighed the risks.

Women may be more likely to experience side effects from the Pfizer-BioNTech and Moderna vaccines. A CDC study published in April found that women reported 79 percent of the side effects from those vaccines but received 61 percent of administered doses.

The vaccines also can cause women’s lymph nodes to swell under the arm where they got their shot. Since this enlargement can be seen on a mammogram and be confused for a sign of breast cancer, the Society of Breast Imaging recommends scheduling your mammogram before your first vaccine dose or at least four weeks after your second dose.

Fertility patients scheduled to receive certain treatments, including oocyte retrieval, embryo transfer and intrauterine insemination, are advised to avoid getting vaccinated within three days before or after their procedures, according to the American Society for Reproductive Medicine. Side effects from the vaccine could make it hard for doctors to recognize any potential issues related to the fertility treatment.

The CDC suggests consulting your doctor about whether you might benefit from taking an over-the-counter medicine, such as ibuprofen or aspirin, after — but not before — vaccination for any pain and discomfort from the shots.

Other recommendations include exercising your arm and applying a cool, wet washcloth over the area where you got the shot. To reduce discomfort from fever, the CDC suggests drinking lots of fluids and dressing lightly.

While side effects are normal, the CDC recommends contacting your doctor if redness or tenderness at the injection site gets worse after 24 hours, if your side effects concern you or if your side effects do not diminish after a few days.

The two-shot Pfizer-BioNTech vaccine has been shown to be 91 percent effective at six months of follow-up. Out of 927 cases of covid-19 with symptoms in the trial, 850 cases were in the placebo group. It was 95 percent effective against severe illness. Data from the real-world use of the vaccine has suggested there is a level of protection after the first shot.

Moderna’s vaccine was found to be 94 percent effective at preventing disease in its trial and particularly effective against severe disease.

The Johnson & Johnson vaccine was found to be 72 percent effective at preventing cases of moderate to severe illness in the United States and was particularly effective against severe disease.

Scientists don’t yet know how long the protection from the vaccines lasts. And they don’t know whether the shots prevent people from becoming infected without knowing it and inadvertently spreading the virus, although there is growing evidence that the vaccine also protects against infection without symptoms.

The efficacy of the Johnson & Johnson vaccine held up against a variant that has raised concerns it could evade immunity — the one first detected in South Africa. The efficacy was 72 percent in the United States vs. 64 percent in South Africa. The efficacy against severe disease was equally as robust against variants as it was against the normal circulating strain.

Moderna has said its vaccine protects against this variant but added the caveat that the vaccine-elicited antibodies were less effective at neutralizing this mutation in a laboratory dish. Pfizer and BioNTech released their own study, not yet peer-reviewed, finding that their vaccine was slightly less effective against the variant first found in South Africa but still effectively neutralizes it.

Drug companies are already working on revising vaccines so that they could be deployed to block the variant of greatest concern. Even if a vaccine is not completely effective against a variant, it is likely to blunt the severity of illness and remain a powerful tool against the pandemic.

Children were not included in the initial coronavirus vaccine trials, but all the vaccines are gradually being tested in younger age groups. In May, the FDA authorized the Pfizer-BioNTech vaccine for use in children as young as 12. A trial in younger children is ongoing and will step down in age to babies as young as 6 months.

On May 25, Moderna announced that its vaccine produced the same immune response in teenagers as adults. The company said it plans to submit its data to federal regulators in early June.

Johnson & Johnson has begun testing its vaccine in older teens. AstraZeneca began a trial in children as young as 6 in Britain, but paused the trial in April until regulators finish reviewing a possible link to rare but potentially dangerous blood clots. Health officials have said the United States could start administering vaccines to children younger than 12 by early next year.

The follow-on trials do not take as long as the initial trials, in which researchers waited to see if people who received vaccines were less likely to fall ill. Instead, companies will find a safe dose for children and then determine whether the immune response triggered by the vaccine is equivalent to that found in adults.

Historically, vaccines have taken years to develop. Before these, the mumps vaccine — which took four years to develop — was the fastest to be approved for use in people. Developing messenger RNA vaccines such as the Pfizer-BioNTech and Moderna products has been fast because scientists were able to start their work before there was a known case of the novel coronavirus in this country, using the viral genome shared online as a template. They were able to draw on years of research developing the vaccine platform.

The Johnson & Johnson vaccine was rapidly designed using the genome of the virus. That vaccine technology had been in development for years against HIV, Ebola and the Zika virus.

The Oxford-AstraZeneca vaccine was 76 percent effective in its large U.S. trial, and the company said it will file for emergency-use authorization from the Food and Drug Administration this spring. The European Union’s regulator has recommended authorization of the vaccine for use in the 27-country bloc, but there is an ongoing investigation into rare but potentially fatal brain blood clots in younger people who have received the shot. Regulators have said the benefits of the vaccine outweigh the risk while the probes continue.

A coronavirus vaccine made by Maryland biotech company Novavax — which is in the midst of a large late-stage trial in North America — proved effective at stopping symptomatic infections in global hot spots where concerning variants are dominant, the company announced in January. But warning signs lurked amid those findings: The protective effect of the vaccine was substantially muted in South Africa, where a worrisome virus variant is in wide circulation. The company has said it will not seek emergency-use authorization until July at the earliest.

The first vaccine authorized in the United States was developed by pharmaceutical giant Pfizer and the German firm BioNTech. It consists of two doses, given three weeks apart. Moderna’s vaccine, developed by the Massachusetts biotechnology company in partnership with the National Institute of Allergy and Infectious Diseases, was the second vaccine cleared. It consists of two doses, given 28 days apart.

The Pfizer-BioNTech and Moderna shots are the first vaccines using messenger RNA technology approved for human use by the FDA. The approach is different from that of more traditional vaccines, which often use a weakened or dead version of a virus, or a laboratory-generated protein. It uses a synthesized scrap of genetic information that is wrapped in a protective fat layer to keep it from disintegrating. When it goes into cells in the muscle of the upper arm, its molecular instructions tell your cells to create a protein that triggers an immune response to the spike protein on the surface of the coronavirus. This means your immune system will be prepared to eliminate the pathogen if it tries to invade.

The Johnson & Johnson vaccine is the first one-shot regimen. It contains cold viruses that have been genetically tweaked so that they cannot make copies of themselves. When those harmless cold viruses infect cells, they deliver the genetic blueprint of the coronavirus spike protein, and the cell learns to build the spike. The immune system learns to recognize and block the real virus.

All adults in the United States have been eligible for vaccination since April. Roughly 17 million children, age 12 and older, became eligible in May, when the FDA granted emergency-use authorization to the Pfizer-BioNTech vaccine for that age group. For the most up-to-date information on vaccine eligibility, see The Washington Post’s vaccine distribution tracker.

The vaccine from Pfizer and BioNTech must be kept ultracold, at minus-70 degrees Celsius during shipping. The companies have created their own GPS-tracked coolers filled with dry ice to distribute it. The vials can be stored for up to two weeks at minus-15 to minus-25 degrees Celsius, conditions that are found in less-specialized pharmaceutical freezers.

Each vial of the Pfizer vaccine holds up to six doses when diluted.

The Moderna vaccine is stored frozen at minus-20 degrees Celsius, but it keeps for a month at refrigerator temperatures. This could make it easier to distribute to pharmacies and rural areas that don’t have specialized freezers.

The Johnson & Johnson vaccine can be kept at refrigerator temperatures for three months. This greatly simplifies the logistics in distributing the vaccine.

The federal government has purchased hundreds of millions of doses with taxpayer money and made them available free, regardless of a person’s health insurance or immigration status.

Unlike a few months ago, the United States now has enough vaccine that many people can choose to visit a vaccination site with the brand they want. Still, if you have already received an FDA-authorized vaccine, the CDC currently does not recommend getting another. Experts also caution that comparing the vaccines’ efficacy numbers is not a straightforward way to measure each option’s value.

The CDC says vaccination should be offered to people regardless of whether they had a previous infection. We are still learning how long immunity to the coronavirus lasts after an infection or a vaccination. There is some evidence that vaccines trigger a stronger immune response than recovering from an infection.

It’s best to wait to be inoculated until you have recovered from your illness and have met all the criteria to end self-isolation to avoid spreading the infection to others. There is no recommended minimum interval to get vaccinated after you’ve recovered.

Pregnant women are at increased risk of severe illness from covid-19 and may want to talk with their medical providers beforehand to make a decision about vaccination. Pregnant women were excluded from the initial coronavirus vaccine trials, so data is limited, although developmental and reproductive animal studies showed no safety concerns. The first trial of a coronavirus vaccine in pregnant women began in February, and more are planned.

The American College of Obstetricians and Gynecologists says a conversation with a clinician may be helpful but should not be required. Pregnant people who get infected with the coronavirus are at greater risk of death and severe illness than those who are not pregnant, even as the overall risk remains small.

Studies show that pregnant women pass along immunity to their babies, in utero and during breastfeeding.

The Pfizer-BioNTech and Moderna vaccines do not contain live virus or any enhancers to boost an immune response and are not thought to be a risk to the breastfeeding infant. They do not alter human DNA in people who get it and cannot cause genetic changes. Also, this type of vaccine breaks down quickly and doesn’t enter the nucleus of the cell.

People who have mild allergies to food, pets, environment or latex can get the Pfizer-BioNTech, Moderna or Johnson & Johnson vaccines.

Concerns about rare but severe allergic reactions were sparked by reports that two health-care workers in Britain had such reactions after being vaccinated with messenger RNA vaccines.

Government officials and medical experts say these rare reactions should not deter the general public from taking the vaccine, although the standard guidance is that people who get the shot should linger at the clinic or doctor’s office for 15 minutes, and 30 minutes if they have a history of severe allergic reactions. Anaphylaxis can be quickly reversed with epinephrine and other medicines.

The FDA and the CDC say people with a history of severe allergic reactions to any component of the vaccines should not get the shots.

Researchers do not know which component in the Pfizer-BioNTech vaccine might have triggered the severe allergic responses. Many, but not all, of the chemicals in that shot and the Moderna vaccine are the same. They both use polyethylene glycol, a chemical widely used in medicines, cosmetics and other household products, which the FDA has said it is looking at as a possible culprit.

If you have a history of severe allergic reaction to another vaccine or injectable therapy, that should not necessarily preclude you from getting the shots. But the CDC advises consulting with your doctor beforehand.

Neither the federal government nor manufacturers of the authorized vaccines have done comprehensive studies of how the shots work in immunocompromised people, who also were not included in the trials. Individuals with compromised immune systems aren’t believed to be at increased risk of adverse reactions to the vaccines. But research into how effectively the vaccines protect patients with various immunosuppressive conditions from getting covid-19 has yielded mixed and at times conflicting results.

Early data suggests that the vaccines do offer some protection for most patients with HIV and autoimmune conditions such as rheumatoid arthritis, although perhaps to a lesser degree than for healthy individuals. But there is concern about some people with certain conditions, such as specific blood cancers or organ-transplant recipients, because emerging research shows that 15 to 80 percent of that population is generating few antibodies after vaccination. This weakened response appears to be at least partly related to certain immunosuppressive drugs and potentially to a commonly prescribed steroid.

In the absence of studies on possible solutions, including booster or high-dose shots of the vaccines, experts are encouraging immunocompromised people to talk with their doctors before getting vaccinated. And post-vaccination, it’s recommended to continue practicing social distancing and other safety precautions.