The first images of people receiving vaccines to prevent COVID-19 brought hope to millions around the world. Health care workers and the elderly were among the first to be vaccinated, and state and local governments are determining who will be next in line.

As the general public awaits its turn for a vaccine, here are answers to some common questions about the COVID-19 vaccines.

What vaccines are currently available in the United States?

There are currently two vaccines to prevent COVID-19 available for emergency use in the United States, one by Pfizer-BioNTech and another by Moderna.

The Pfizer-BioNTech vaccine is given via two shots in the muscle in the upper arm administered 21 days apart. The vaccine was found to be 95% effective at preventing COVID-19 in people during clinical trials. The CDC recommends this vaccine for people 16 and older.

The Moderna vaccine is also given via two shots in the muscle in the upper arm, but unlike the Pfizer vaccine, the second dose is given 28 days after the first. Clinical trials found the Moderna vaccine to be 94.1% effective. The CDC recommends this vaccine for people 18 and older.

Three other COVID-19 vaccines are in Phase 3 clinical trials in the U.S.  One of these vaccines requires only one shot.

How do the COVID-19 vaccines work? 

The Pfizer-BioNTech and the Moderna vaccines are a new type of vaccine known as messenger RNA vaccines (mRNA). Unlike many other vaccines, mRNA vaccines do not use a live or weakened virus to build immunity. That means you cannot get COVID-19 from an mRNA vaccine. Instead, these vaccines teach our body’s cells how to make a protein — or part of a protein — that triggers an immune response. That immune response produces antibodies which protect us from getting sick if we are exposed to the real virus.

Scientists have been studying mRNA vaccines for decades, so when COVID-19 began spreading they were able to begin working on a vaccine immediately. This technology holds great potential. In the future, mRNA vaccines could be used to prevent multiple diseases with a single shot, eliminating the need for multiple vaccinations. Cancer researchers are also using mRNA to trigger the immune system to target specific cancer cells.

What are the possible side effects? 

It is completely normal to have some side effects after receiving a COVID-19 vaccine. It is a sign the vaccine is working. Some side effects may prevent you from doing some everyday activities but will go away in a few days.

The most common side effects include pain and swelling in the arm where you got the shot, and headache, fever, chills and tiredness throughout the rest of the body. Your doctor can recommend over-the-counter medications to relieve pain or discomfort. Even if you experience side effects, it is important to get the second shot unless your doctor or vaccination provider advises against it.  Both shots are needed to get the most protection.

Some people have suffered severe allergic reactions (anaphylaxis) after getting a COVID-19 vaccine. If you have a history of severe allergic reactions, it’s a good idea to review the vaccine ingredients with your doctor. The CDC recommends not getting the vaccine if you have had a severe allergic reaction to any ingredient in an mRNA vaccine or if you suffer a severe allergic reaction after receiving the first dose.

Do I really need my second dose?

 As we said above, both the Pfizer-BioNTech and Moderna vaccines require two shots for maximum protection against COVID-19. You may have heard that British health officials decided to delay second doses in order to give a first dose to as many people as possible amid a dramatic surge in COVID-19 cases. The U.S. does not support this strategy. The Food and Drug Administration warned delaying the second dose could be dangerous and advised U.S. health care providers to stick to the current vaccine schedule.

Do I still need to wear a mask after getting the vaccine? 

Yes. It is important to continue to follow public health advice after you receive your vaccine. We are still learning how these vaccines work in real-world conditions, so for now we must use the tools available to us — such as wearing a face mask, staying at least six feet away from others and washing hands frequently — to stop this pandemic. There are several reasons for this.

First, it typically takes a few weeks for the body to build immunity against a virus after vaccination, so it is possible to get infected with COVID-19 just before or after the vaccination and still get sick because the vaccine has not had enough time to work.

Second, while the vaccine will probably prevent you from getting seriously ill from COVID-19, scientists are still trying to determine if vaccinated people can still spread COVID-19 to others.

The number of people vaccinated and the number of cases of COVID-19 spreading through a community will also impact when health officials determine it is safe to stop wearing face masks and social distancing.

If I recovered from COVID-19 do I still need to get the vaccine when it becomes available? 

Yes. The CDC recommends people get the vaccine even if they have had a COVID-19 infection due to the severe health risks associated with the virus. Experts believe people develop some degree of natural immunity from an infection, but it varies from person to person and it is unclear how long the immunity will last.