Your vaccine questions, answered: I had COVID, should I still get vaccinated? What are the side effects? What are its 'ingredients?'

The Food and Drug Administration on Friday authorized the first coronavirus vaccine for emergency use, and distribution of Pfizer's vaccine was underway over the weekend.

Moderna has also applied for emergency use authorization for its two-shot vaccine candidate, and more companies are expected to apply in the coming months. Meanwhile this week, Canada and the United Kingdom also authorized the widespread use of Pfizer's vaccine.

As vaccines are being produced in record time, what do we know about these shots? What are the side effects? Will you be immune? And will you have to take the vaccine once or every year?

We know you have questions, and we’re here to help. Ask us your vaccine questions through this online form, and we'll speak with public health experts to answer them below.

Here's what we know:

Is there a vaccine for the coronavirus?

There are more than 200 vaccine candidates under development, with 52 in clinical trials, according to the World Health Organization. As of late November, Phase 3 clinical trials were in progress or being planned for five vaccines in the U.S., according to the U.S. Centers for Disease Control and Prevention.

The U.S., United Kingdom and Canada have authorized the use of a vaccine developed by drug companies Pfizer and BioNTech. China and Russia began rolling out their own vaccines before completing late-stage clinical trials.

Pfizer’s COVID-19 vaccine: Key committee endorsement paves way for clearance by FDA and vaccinations in US to begin

Is the vaccine safe?

The U.S. Food and Drug Administration on Tuesday released a 53-page report summarizing data from Pfizer and BioNTech's COVID-19 candidate vaccine trial. The data supports earlier findings that the vaccine is safe and will prevent 95% of people from becoming sick with COVID-19. Read more.

If you already had COVID-19, should you still get vaccinated?

People who have had COVID-19 "may be advised" to get the vaccine, "due to the severe health risks associated with COVID-19 and the fact that re-infection with COVID-19 is possible," according to the CDC.

If you've had COVID-19 and recovered, you have what's called natural immunity. Studies have shown natural immunity to COVID-19 may last months to years. Immunity from a vaccine is called vaccine-induced immunity, and it's unclear how long that may last.

Will the vaccine be safe for children and pregnant women?

Historically, major vaccines have not been tested during pregnancy because of concerns that both the pregnant person and fetus would be at risk for complications. Pregnant and nursing people were not included in the Pfizer/BioNTech study, so there is no data to suggest whether they should avoid vaccination.

Dr. Peter Marks, who directs the Center for Biologics Evaluation and Research at the FDA, said Saturday that pregnant people, children and those who are immunocompromised should discuss vaccination with their providers and "will need to consider on an individual basis."

In a Dec. 1 meeting of a CDC group that determines how vaccines should be allocated, several expert members said they expected the vaccine would be safe for mothers who are nursing because the virus itself does not seem to pass through the mother’s milk.

The Pfizer trial data only included children ages 16 and up, though the study has since been extended to age 12, and will include younger children as soon as the company decides what dose it wants to test in this group. Read more.

Related: No, the COVID-19 vaccine doesn’t cause infertility in womenmen

What about 16- and 17-year-olds?

Although there's not as much data for this age group as others, officials believe they know enough to conclude the benefits of getting vaccinated outweigh the risks, Marks said.

Sixteen and 17-year-olds rarely get severely ill with COVID-19, but they do transmit it easily, often without showing symptoms. And some in that age group work jobs in which they interact with the public, such as check-out clerks.

What are the side effects of the COVID-19 vaccine?

Americans will likely experience at least one side effect from the COVID-19 vaccine, but doctors say that’s normal and you should still get vaccinated. For the vaccine by Pfizer/BioNTech, many trial participants endured side effects for a day or two after getting their shots, particularly the second one.

The most commonly reported side effect among vaccine recipients under age 55 was a sore arm, followed by fatigue (60% after the second shot); headache (52% after the second shot); other muscle aches (37%); and chills (35%). About 28% took pain medication after the first shot and 45% after the second shot. Read more.

Can you get one dose from one vaccine and the second from another?

The Pfizer/BioNTech and the Moderna vaccines both require two doses, given 21 and 28 days apart, respectively. Since the vaccines differ in composition, storage and time between the two doses, experts say people must take the same vaccine for both doses.

What is the level of immunity after one shot?

The vaccine by Pfizer/BioNTech was 52% effective after the first dose, according to a recent FDA report. However, because everyone received a second dose three weeks after the first, there’s no evidence of protection lasting longer than a few weeks after the first dose. Read more.

How long will a COVID-19 vaccine be protective?

There is no data on how long a vaccine will be protective. FDA officials were expected to authorize Pfizer's vaccine in the midst of the pandemic because it is safe and at least transiently effective, rather than withholding it for two years to await the typical long-term results required for a full approval. Vaccine companies intend to follow trial participants for two years and submit a full application when they have that data. Read more.

Once you get vaccinated, can you still get sick or spread the virus?

Yes, it's possible. The potential vaccines are not 100% effective, so there’s a small chance you could encounter the virus and still get sick. It also typically takes "a few weeks" for the body to build immunity after vaccination, meaning a person could be infected with the virus that causes COVID-19 just before or just after vaccination and get sick, according to the CDC.

Even after vaccination, it's not clear whether someone could catch the virus that causes COVID-19 and pass it on to someone else, even without becoming ill. The studies designed to test the candidate vaccines only examined symptomatic infections, not whether vaccinated people could still be contagious. This means that people who are vaccinated can be pretty sure that they won’t develop COVID-19 themselves, but they could still pass it on to others without knowing they are infected. Future studies will explore this. Read more.

Could the vaccine make subsequent infections worse?

There has been concern that getting vaccinated against the coronavirus might cause a subsequent COVID-19 infection, as some dengue vaccines have been shown to do for that disease.

The FDA concluded that use of the vaccine has not been widespread enough to know for certain, but that "available data do not indicate a risk of vaccine-enhanced disease, and conversely suggest effectiveness against severe disease within the available follow-up period." The risk does remain over time as immunity against the disease wanes, the FDA said, and "needs to be evaluated further in ongoing clinical trials." Read more.

Which vaccines will be only one shot?

Johnson & Johnson is developing a single-dose vaccine. The company's chief scientist said last month that the company expects to have all the data it needs to file for authorization by February or sooner.

What are the COVID-19 vaccine 'ingredients?'

Experts say the ingredients in the COVID-19 vaccine developed by Pfizer/BioNTech looks pretty standard for a vaccine. They can be organized into four basic categories: the active ingredient (messenger RNA, in this case), fats, sugars and salt. Read more on ingredients.

None of the vaccines in development in the U.S. use the "live" virus that causes COVID-19, according to the CDC. Most of the vaccines under development introduce the "spike" protein found on the surface of the virus. They train the immune system to recognize this protein and attack in case of infection. But both the Pfizer and Moderna vaccines depend on a technology never before used in a commercial vaccine: mRNA, or messenger ribonucleic acid. Read more on mRNA.

What if I have had an allergic reaction in the past?

About 1.6% of the population has had a severe allergic reaction to food or something in the environment, but they are eligible to receive the Pfizer/BioNTech vaccine, Marks said. Only people who have had a severe allergic reaction to a previous dose of the Pfizer/BioNTech vaccine or one of its components should not receive the vaccine. Sites where the Pfizer/BioNTech vaccine are administered will be able to treat allergic reactions, Marks said.

Two British people with severe allergies apparently had reactions to Pfizer/BioNTech's vaccine this week. Allergic reactions were not a significant problem in the U.S. trial, in which more than 20,000 people received two doses of the vaccine. The U.S. trials kept out subjects who have had severe allergic reactions.

Where are the Pfizer and Moderna vaccines made?

Pfizer has two plants making its COVID-19 vaccine, one in Kalamazoo, Michigan, and one in Puurs, Belgium. The company has a distribution site in Pleasant Prairie, Wisconsin, where its vaccine is being stored.

Moderna has started producing vaccine at its plant in Norwood, Massachusetts, and it will ramp up production in the next month with help from contract manufacturer Lonza Biologics, which has a facility in Portsmouth, New Hampshire.

How will the vaccine be shipped?

Shipment of the Pfizer/BioNTech vaccine involves Pfizer personnel taking five-dose glass vials of vaccine out of subzero freezers and packing them into specially designed shipping containers, Wes Wheeler, president of Global Healthcare at UPS, told USA TODAY.

Pfizer's COVID-19 vaccine is being shipped in insulated containers that hold between 195 and 975 five-dose vials and are about the size of a carry-on suitcase. The vials are stored in flat, pizza box-sized compartments, each of which holds 195 vials. A fully loaded thermal container, which is reusable, contains five of these and weighs about 70 pounds.

Pfizer will ship its vaccine using UPS and FedEx as its main distributors. Moderna's vaccine distribution is being coordinated by McKesson, the nation's largest medical supply distributor. Each carton will have a GPS tracker allowing it to be located in real-time anywhere in the world, as well as a Bluetooth tracker that can locate it within 10 feet inside a UPS facility, said Wheeler. Read more.

How much vaccine is there?

Approximately 2.9 million doses of the Pfizer/BioNTech vaccine will be distributed across the 50 states in the next few days, Gen. Gustave Perna, co-leader of Operation Warp Speed in charge of logistics, said Saturday. Another 2.9 doses are being held back to distribute later for the second shot, plus reserves.

More vaccine will be rolled out in the weeks and months to come, with Pfizer and Moderna expected to deliver a combined 300 million doses of their vaccines by the middle of next year, enough to vaccinate nearly half the population of the U.S.

What is the vaccine delivery schedule?

The Pfizer/BioNTech vaccine will be sent to to 636 hospitals, clinics and public health systems nationwide this week, with 145 sites receiving doses Monday, 425 on Tuesday and the final 66 on Wednesday, Perna said.

Once the initial doses have shipped, the system will begin regular deliveries. Each Thursday, Operation Warp Speed, the White House's coronavirus vaccine initiative, will ask vaccine producers how much vaccine they have made during the week, Paul Ostrowski, who leads supply, production and distribution for the federal government's Operation Warp Speed, who walked USA TODAY through the process. On Friday, staff will meet and decide how many doses can be allocated. States will put in their orders on Saturday and deliveries will happen on Monday morning, he said.

UPS, FedEx and McKesson all have 24-hour hotlines sites can call if there are delivery troubles. In addition, Operation Warp Speed officials said they will have staff available at two operations centers, one in Washington and one in Atlanta, both with 24-hour hotlines. Read more.

How does Operation Warp Speed track doses?

Operation Warp Speed created a software system called Tiberius that allows states and local jurisdictions to order and track vaccine. It records all allocations of COVID-19 vaccines to states, territories, cities and federal entities, said Col. Deacon Maddox, OWS Chief of Plans, Operations and Analysis. It works in conjunction with the CDC's Vaccine Tracking System, known as VTrckS, which has been distributing vaccine for children for a decade. Read more.

How much does the vaccine cost?

Moderna has agreed to provide the U.S. with 200 million doses and Pfizer at least 100 million by the middle of next year, have agreed to provide the U.S. with 100 million doses apiece, already funded through federal support of manufacturing and distribution. Vaccine doses purchased with U.S. taxpayer dollars will be given to the American people at no cost, according to the CDC.

"However, vaccine providers will be able to charge administration fees for giving or administering the shot to someone. Vaccine providers can get this fee reimbursed by the patient’s public or private insurance company or, for uninsured patients, by the Health Resources and Services Administration’s Provider Relief Fund," the CDC says on its website.

This article originally appeared on USA TODAY: COVID vaccine, explained: Pfizer FDA approval, side effects, Moderna