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How does the COVID vaccine work? Answers to your frequently asked questions.

It’s been nearly a year since the novel coronavirus was declared a pandemic, and there have been millions of U.S. cases and hundreds of thousands of deaths from the COVID-19 illness.

The federal government is distributing two vaccines it authorized for emergency use in December, one from Pfizer-BioNTech and another from Moderna. A third is under review by the U.S. Food and Drug Administration, and more are in the pipeline. Together, they could vastly improve the availability of still-scarce supplies.

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How do vaccines get authorized?

Normally it takes years to secure FDA approval for new drugs and medical devices. Once a product is developed, it must go through three phases of human trials as laid out by the agency to show whether they are safe and effective.

Pfizer-BioNTech and Moderna followed this path for their vaccines, but developed them at a remarkable pace of months instead of years. They were allowed to condense the time frame for the trials given the pandemic, but regulators say the data received the same level of scrutiny.

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The regulators granted both vaccine makers emergency-use authorizations to market their vaccines, a temporary designation that requires them to continue tracking tens of thousands of volunteers for years before they are formally approved.

Other vaccines are expected to follow this path, including one from Johnson & Johnson, which recently submitted data to the FDA for authorization and is expected to be approved this month.

How effective are these vaccines?

The two authorized vaccines are considered extraordinarily effective at preventing illness due to COVID-19, though it’s not yet known how well they prevent someone from transmitting the virus to others.

The Pfizer vaccine was shown to be about 95% effective and the Moderna vaccine was shown to be about 94% effective. Both require two doses to achieve that level. Some scientists have suggested administering one less-effective dose so the vaccine can be distributed to more people. The CDC has not endorsed that strategy, but officials there say delaying the second dose by up to six weeks is OK.

The two vaccines’ levels of effectiveness are in league with the most effective vaccines, including the measles-mumps-rubella vaccine. According to the U.S. Centers for Disease Control and Prevention, that vaccine is 88% to 97% effective after two doses.

They are far more effective than the annual flu vaccine, which is remade each year to match circulating strains. It’s usually about 40% to 60% effective, though scientists say the flu vaccine has been shown to still lessen the severity of illness even if it doesn’t prevent it.

The Johnson & Johnson vaccine isn’t expected to be as effective as the other COVID-19 vaccines, but authorities say it will provide strong protections against severe illness and will be logistically easier to disburse than the other coronavirus vaccines. It requires only one dose and doesn’t need the ultracold storage required by the other two.

How were the vaccines developed and how do they work?

The two authorized vaccines are called messenger RNA, or mRNA, vaccines. They have been studied for years for other purposes such as preventing other viruses and some kinds of cancers. None has been approved.

Vaccine makers, many with a huge infusion of federal money, were able to adapt the method for use against the coronavirus. A handful of U.S. scientists provided vaccine makers with a leg up in that adaptation because they had been studying other coronaviruses, such as SARS and MERS after global outbreaks in 2003 and 2015, respectively.

Some of that research served as a platform for the vaccines now authorized and in development.

Standard vaccines, such as the flu vaccine, deploy a live, weakened bit of virus or dead virus to spark an immune response in the body. The resulting antibodies fight off the real virus when someone later comes into contact with it. Other coronavirus vaccines use variations of this method.

But, the CDC explains, mRNA vaccines instead instruct cells to trigger the immune response.

They do this by using strands of genetic material to tell cells to make a harmless spike protein found on the surface of the virus. The body’s immune system sees the protein as an invader and makes antibodies to fend off the threat.

What are the common side effects and risks from the vaccine?

Doctors say many people will develop some symptoms after receiving a shot, especially the second dose. They can include fatigue, chills, a low-grade fever and soreness at the vaccination site.

The symptoms typically go away after eight to 12 hours. Doctors say to plan for some time to rest because it’s not clear who will be most heavily affected. And have some acetaminophen, known as Tylenol, on hand.

Those who experience shortness of breath or a rash should seek medical attention. Rarely, a few people have had allergic reactions to the vaccine.

There is no risk of getting the COVID-19 disease from the vaccine.

Full protection takes about two weeks from the second dose, and federal authorities say people should still wear masks and follow other precautions through that time and until most of the population is vaccinated.

Do the vaccines work against the newly circulating variants?

Labs around the country are increasingly finding variants of the coronavirus that are far more contagious. One first identified in the United Kingdom, known as B. 1.1.7, is now expected to be the dominant strain by the end of March.

So far, federal health authorities say, the vaccines appear to still be effective against this variant.

The vaccines, however, might be less effective against a variant first found in South Africa but also starting to surface during lab testing in the United States.

Officials say it’s something of a race to see whether the vaccine shots can’t get into arms fast enough to stem infections. Otherwise, there could be a huge surge in cases.

That prospect led the CDC to recommend that people double down on preventive measures. The officials also recommended people tighten their masks on their noses and under their chins to reduce air leakage or double up on masks by wearing a surgical mask under a cloth mask to block more of the virus.

What if the vaccines do stop preventing illness?

Scientists say mutations were expected and not always a threat. But they do mean that the virus might not fully disappear.

Like other viruses that cause respiratory disease, the coronavirus could require an annual vaccine to match circulating strains, similar to the flu vaccine.

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