Answers to your questions about the new COVID vaccines in the U.S.

Coronavirus vaccines are rolling out to health workers now and are expected to reach the arms of most of the rest of us by spring. Here are answers to some questions about the vaccines and the vaccination process.

This is the expected order:

— Health care workers and people in long-term care facilities. The 21 million health care workers and 3 million mostly older people in long-term care facilities are going first, starting this month.

— Front-line essential workers and people 75 and older.

— Other essential workers, adults with medical conditions and people over 65. The Centers for Disease Control and Prevention is recommending all remaining at-risk adults and the remaining 57 million workers in food and agriculture, manufacturing, law enforcement, education, transportation, corrections, emergency response and other sectors considered essential by the government, be prioritized for a third wave of vaccination at some point this winter. States again will have the final say on who is in this group.

— All other adults. Adults in the general population are at the back of the line. They could start receiving the vaccine as early as April, said Dr. Anthony Fauci, the nation’s top infectious disease expert, although many people are likely to have to wait until at least May or June. The vaccine hasn’t been approved in children, so it may be several months, or possibly a year, before a vaccine is available for anyone under the age of 16.

There aren’t enough doses, so initially the vaccine will be rationed for those who need it most. It will take time to produce and distribute the vaccine, and then schedule two vaccinations per person, three to four weeks apart. As more vaccines get approved, things will speed up. At least 40 million doses (enough for 20 million people) should be available from Pfizer and Moderna by the end of the year, and much more will come in 2021.

— ABBY GOODNOUGH

Hospitals and medical groups are contacting health workers to schedule vaccine appointments. FedEx and UPS will transport the vaccine throughout most of the country, and each delivery will be followed by shipments of extra dry ice a day later.

Pfizer designed special containers, with trackers and enough dry ice to keep the doses sufficiently cold for up to 10 days. Every truck carrying the containers will have a device that tracks its location, temperature, light exposure and motion. Pfizer will ship the special coolers, each containing at least 1,000 doses, directly to locations determined by each state’s governor. At first, almost all of those sites will probably be hospitals that have confirmed that they can store shipments at minus 94 degrees Fahrenheit (minus 70 Celsius), as the Pfizer vaccine requires, or use them quickly.

The pharmacy chains CVS and Walgreens have contracts with the federal government to send teams of pharmacists and support staff into thousands of long-term care facilities in the coming weeks to vaccinate all willing residents and staff members. CVS and Walgreens are both planning to administer their first vaccinations as early as this week. More than 40,000 facilities have chosen to work with CVS. Nearly 35,000 picked Walgreens. Each state has already picked, or will soon pick, either the Pfizer or the Moderna vaccine for its long-term care facilities that will be working with the pharmacies.

— ABBY GOODNOUGH and TARA PARKER-POPE

It’s quite likely that when the general public starts getting vaccinated in April, shots will be scheduled through doctors’ offices, CVS, Walgreens and other pharmacies — the same way people get flu shots. However, final plans will depend on what other vaccines besides Pfizer’s and Moderna’s have been approved.

— ABBY GOODNOUGH and TARA PARKER-POPE

You should not have to pay anything out of pocket to get the vaccine, although you will be asked for insurance information. If you don’t have insurance, you should still be given the vaccine at no charge.

Congress passed legislation this spring that bars insurers from applying any cost sharing, such as a copayment or deductible. It layered on additional protections barring pharmacies, doctors and hospitals from billing patients, including those who are uninsured. Even so, health experts do worry that patients might stumble into loopholes that leave them vulnerable to surprise bills. This could happen to those who are charged a doctor visit fee along with their vaccine, or Americans who have certain types of health coverage that do not fall under the new rules.

If you get your vaccine from a doctor’s office or urgent care clinic, talk to them about potential hidden charges. To be sure you won’t get a surprise bill, the best bet is to get your vaccine at a pharmacy once the shots become available.

— SARAH KLIFF

Nobody’s giving away your shot! If you are in the first group of people vaccinated, your booster shot will be set aside for you and won’t be given to someone else. Later when supplies are more plentiful, reserves probably won’t be necessary. Gen. Gustave F. Perna, the chief operating officer for Operation Warp Speed, the federal effort to speed a vaccine to market, has said that after the first doses are given, the doses earmarked for the second shot will be set aside to be given three weeks later. An additional 500,000 backup doses will also be held in reserve, in case they are unexpectedly needed.

“Eventually, we will become much more confident in our manufacturing, our distribution process, state handling, et cetera,” Perna said. “And then the requirement for reserve won’t be necessary.”

This depends on a number of factors, including the supply in your area at the time you are vaccinated and whether certain vaccines are found to be more effective in certain populations, such as older adults. Pfizer’s vaccine was the first to be authorized for use, followed by Moderna’s late last week.

— ABBY GOODNOUGH and TARA PARKER-POPE.

The injection into your arm won’t feel different from any other vaccine, but the rate of short-lived side effects does appear higher than a flu shot. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. The side effects, which can resemble the symptoms of COVID-19, last about a day and appear more likely after the second dose. Early reports from vaccine trials suggest some people might need to take a day off from work because they feel lousy after receiving the second dose. In the Pfizer study, about half developed fatigue. Other side effects occurred in at least 25-33% of patients, sometimes more, including headaches, chills and muscle pain.

While these experiences aren’t pleasant, they are a good sign that your own immune system is mounting a potent response to the vaccine that will provide long-lasting immunity.

— ABBY GOODNOUGH and TARA PARKER-POPE

Each company’s application to the Food and Drug Administration includes two months of follow-up safety data from Phase 3 of clinical trials conducted by universities and other independent bodies. In that phase, tens of thousands of volunteers got a vaccine and waited to see if they became infected, compared with others who received a placebo. By September, Pfizer’s trial had 44,000 participants; no serious safety concerns have been reported.

— ABBY GOODNOUGH

People with severe allergies who have previously experienced anaphylaxis should talk to their doctors about how to safely get the vaccine and what precautions to take. Although severe reactions to vaccines are rare, two health care workers had anaphylaxis after receiving the vaccine on the first day it became available in Britain. Both workers, who had a history of severe reactions, were treated and have recovered. (Anaphylaxis can be life-threatening, with impaired breathing and drops in blood pressure that usually occur within minutes or even seconds after exposure to a food, medicine or substance like latex.)

For now, British authorities have said the vaccine should not be given to anyone who has ever had an anaphylactic reaction. U.S. health experts have said such warnings are premature because severe reactions can be treated or prevented with medications. After the British cases, the FDA said it would require Pfizer to increase its monitoring for anaphylaxis and submit data on it once the vaccine comes into use. Fewer than one in 1 million recipients of other vaccines a year in the United States have an anaphylactic reaction, said Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia.

Among those who participated in the Pfizer trials, a very small number of people had allergic reactions. A document published by the FDA said 0.63% of participants who received the vaccine reported potential allergic reactions, compared with 0.51% of people who received a placebo. In Pfizer’s late-stage clinical trial, one of the 18,801 participants who received the vaccine had an anaphylactic reaction, according to safety data published by the FDA. None in the placebo group did.

Once a vaccine starts to reach large numbers of people, it’s possible (and not uncommon) for a small number of severe “adverse events” to occur. Many existing vaccines, including the flu shot, also can cause rare complications, including Guillain-Barré syndrome, seizures and sudden unexplained death. While this sounds frightening, the risk is minuscule when considered over the millions of people who are safely vaccinated each year, and some of these complications can be triggered by the virus itself. Health officials will investigate each event to see if it’s simply coincidence — or if it could have been caused by the vaccine.

While everyone should be prepared to hear about these reports, they should not be a cause for worry or prompt you to delay getting the vaccine. Your risk of severe complications from COVID-19 is far higher than your risk of complications from the vaccine.

It’s safe, and probably even beneficial, for anyone who has had COVID to get the vaccine at some point, experts said. Although people who have contracted the virus do have immunity, it is too soon to know how long it lasts. So for now, it makes sense for them to get the shot. The question is when. Some members of the CDC advisory committee have suggested people who have had COVID in the past 90 days should be toward the back of the line.

All the evidence we have so far suggests that the answer is yes. The clinical trials for the two leading vaccines have shown that they work about the same in older people as in younger people. As the vaccines get distributed, the vaccine makers and the CDC will continue to monitor the effectiveness of the vaccine in people 65 and older who, because of age-related changes in their immune systems, often don’t respond as well to vaccination as younger people do. But just as certain flu vaccines have been developed to evoke a stronger immune response in older people, it’s possible that one of the new vaccines could emerge as a better option for this age group. It’s just far too soon to know.

— CARL ZIMMER

COVID-19 is by far the more dangerous option, even if you are young and at low risk. Although people who are older, obese or have other health problems are at highest risk for complications from COVID-19, younger people can become severely ill, too. In a study of more than 3,000 people ages 18-34 who were hospitalized for COVID, 20% required intensive care and 3% died.

And as many as 1 in 3 people who recover from COVID have chronic complaints, including exhaustion, a racing heart and worse for months afterward. COVID vaccines, in contrast, carry little known risk.

Pregnant and breastfeeding women should consult with their obstetricians and pediatricians about whether to get the vaccine. The Pfizer vaccine has not been tested in pregnant women or in those who were breastfeeding, and federal health officials have not issued any specific guidance, other than allowing these women to be vaccinated if they choose.

In the initial rollout, it will mostly be pregnant health care workers who must weigh the benefits and possible risks. By the time the vaccine is available to pregnant essential workers or to women in the general population, there should be a lot more data available.

Some experts said the virus itself posed greater risks to pregnant women than the new vaccine. Since the 1960s, pregnant women have been urged to receive vaccines against influenza and other diseases. These women are generally cautioned against live vaccines, which contain weakened pathogens — but the Pfizer vaccine does not contain live virus.

— APOORVA MANDAVILLI

A false claim has been circulating online that the new vaccine will threaten women’s fertility by harming the placenta. Here’s why it’s not true.

The claim stems from the fact that the vaccines from Pfizer and Moderna cause our immune systems to make antibodies to something called a “spike” protein on the coronavirus. The false warnings about fertility are based on the claim that these antibodies could attack a similar protein that is made in the placenta during pregnancy, called syncytin. In reality, the spike protein and syncytin are similar only in one very small region; there’s no reason to believe antibodies that can grab onto spike proteins would lock onto syncytin.

Also, the human body generates its own supply of spike antibodies when it fights off the coronavirus, and there’s no sign that these antibodies attack the placenta. If they did, you would expect that women who got COVID-19 would suffer miscarriages. But a number of studies show that COVID-19 does not trigger miscarriages.

— CARL ZIMMER

So far, no coronavirus vaccine has been approved for children. New vaccines are typically tested on adults before researchers begin trials on children, and coronavirus vaccine developers are following this protocol. In September, Pfizer and BioNTech began studying their vaccine on children as young as 12. Moderna followed suit in December. If these trials yield good results, the companies will recruit younger children. The FDA will then have to review these results before the vaccines can get emergency authorization.

Vaccines are typically tested on adults first in the interest of safety. But once a vaccine is shown to be safe and effective in adults, researchers have to run more trials on children to adjust the dosage for their bodies. Another factor in the wait for a vaccine for children is that they are far less likely to die from COVID-19 than adults are. The CDC issued a report in September that concluded that, of more than 190,000 people who died in the United States with COVID-19, only 121 were under age 21.

The Pfizer-BioNTech and Moderna vaccines both have two doses, with the booster shot coming a few weeks after the first. Pfizer-BioNTech’s second dose comes three weeks after the first, and Moderna’s comes four weeks later. The second dose provides a potent boost that gives people strong, long-lasting immunity.

If for some reason you fail to get the second shot precisely three weeks after the first, you don’t have to start over with another two-dose regimen. “The second dose can be picked up at any time after the first. No need to start the series over,” said Offit of the University of Pennsylvania, who is a member of the FDA’s vaccine advisory panel.

And while the two leading vaccines include a second dose, some future vaccine candidates may only require one dose. Johnson & Johnson, for example, is expecting data in January that will show whether its experimental vaccine works after a single dose. In case it doesn’t, the company has also started a separate trial using two doses.

— CARL ZIMMER and TARA PARKER-POPE

Yes, but not forever. Here’s why. The coronavirus vaccines are injected deep into the muscles and stimulate the immune system to produce antibodies. This appears to be enough protection to keep the vaccinated person from getting ill. But what’s not clear is whether it’s possible for the virus to bloom in the nose — and be sneezed or breathed out to infect others — even as antibodies elsewhere in the body have mobilized to prevent the vaccinated person from getting sick.

The vaccine clinical trials were designed to determine whether vaccinated people are protected from illness — not to find out whether they could still spread the coronavirus. Based on studies of flu vaccine and even patients infected with COVID-19, researchers have reason to be hopeful that vaccinated people won’t spread the virus, but more research is needed. In the meantime, everyone — even vaccinated people — will need to think of themselves as possible silent spreaders and keep wearing a mask.

— APOORVA MANDAVILLI

Employers do have the right to compel their workers to be vaccinated once a vaccine is formally approved. Many hospital systems, for example, require annual flu shots. But employees can seek exemptions based on medical reasons or religious beliefs. In such cases, employers are supposed to provide a “reasonable accommodation” — with a coronavirus vaccine, for example, workers might be allowed to work if they wear a mask, or work from home.

— ABBY GOODNOUGH

The test positivity rate in your community will be an indicator of how things are going. This number is the percentage of overall tests given in a community that come back positive. The lower the number, the fewer new cases and the less likely you are to cross paths with someone who has the virus. “The best number is zero,” Fauci said. “It’s never going to be zero, but anywhere close to that is great.”

— TARA PARKER-POPE

Public health officials estimate that 70-75% of the population needs to be vaccinated before people can start moving freely in society again. If things go well, life could get a lot better by late spring and early summer.

Given the surveys so far showing significant public reluctance to get vaccinated, however, it may take a while to see widespread community protection, Fauci said, “If it turns out that only 50% get vaccinated, then it’s going to take much, much longer to get back to the kind of normality that we’d like to see.”

They will be much less effective at preventing death and illness in 2021 if they are introduced into a population where the virus is raging — as is now the case in the United States. A vaccine that’s 95% effective, as Moderna’s and Pfizer’s versions appear to be, is a powerful fire hose. But the size of a fire is still a bigger determinant of how much destruction occurs.

According to the authors of a paper in the journal Health Affairs, at the current level of infection in the country (about 200,000 confirmed new infections per day), a vaccine that is 95% effective — distributed at the expected pace — would still not be enough to end the terrible toll of the virus in the six months after it was introduced. Almost 10 million or so Americans would contract the virus, and more than 160,000 would die.

Measures that reduce the virus’ spread, like mask wearing, social distancing and rapid-result testing, can still have profound effects. Public health officials hope that people will continue to take these precautions at least until the country reaches a vaccination rate of 70-75%.

In the coming weeks, major airlines including United, JetBlue and Lufthansa plan to introduce a health passport app, called CommonPass, that aims to verify passengers’ coronavirus test results — and perhaps soon, vaccinations. CommonPass notifies users of local travel rules — like having to provide proof of a negative virus test — and then aims to check that they have met them.

Although no plans are in place yet to require proof of vaccination for travel or other activities, electronic vaccination credentials could have a great effect on efforts to control the virus and restore the economy. They could prompt more employers and college campuses to reopen. Developers say they may also give some consumers peace of mind by creating an easy way for movie theaters, cruise ships and sports arenas to admit only those with documented virus vaccinations.

— NATASHA SINGER

That is to be determined. It’s possible that coronavirus vaccinations will become an annual event, just like the flu shot. Or it may be that the benefits of the vaccine last longer than a year.

The Pfizer-BioNTech and Moderna vaccines use a genetic molecule to prime the immune system. That molecule, known as mRNA, is 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 years, their mRNA does not — it is destroyed by our cells within days. While these are described as “genetic” vaccines, the vaccines don’t alter your genes in any way.

— CARL ZIMMER

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 to come down with COVID-19 symptoms and then get a positive test. Among those who got sick, 162 had received a placebo shot; just eight had received the real vaccine. This result shows that receiving a vaccine drastically lowered the chances of getting COVID-19 compared with receiving a placebo.

The difference is expressed as efficacy: The Pfizer-BioNTech vaccine has an efficacy rate of 95%. (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%.) 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

Pfizer did not accept federal money to help develop or manufacture the vaccine, unlike the 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 it delivers the vaccines.

— CARL ZIMMER and KATIE THOMAS

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 has an efficacy rate of 94.5%, 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 from 60-90%, depending on the strength of the doses that volunteers received. 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 reformulating its vaccine for new trials.

— CARL ZIMMER and KATIE THOMAS