The arrival of the first COVID-19 vaccine will not mean people can throw away their face masks, experts say.

Face coverings, social distancing and a lot of handwashing still will be required for some time after COVID-19 vaccines become available. But for how long? That depends on how effective the vaccines are and how long protection lasts. Such questions will not be answered when the first injections arrive.

"This vaccine is not likely to be a suit of armor," said William Schaffner, an expert on infectious disease at Vanderbilt University Medical Center in Tennessee.

In other words, a vaccine is not likely to be 100 percent effective. Seasonal influenza vaccines, for example, have been 60 percent effective, at best, in recent years. The least efficient vaccines were only 10 percent effective.

The United States Food and Drug Administration, FDA, has said it aims to have a 50 percent rate of effectiveness in COVID-19 vaccines.

A partly effective vaccine might help high-risk groups, such as healthcare workers or those with existing health problems. But it might not be right for the general population, noted Jesse Goodman, an infectious disease expert and former Chief Scientist at the FDA.

"...That's not the vaccine you would ideally immunize 300 million people with," he said. "Even if everybody was immunized, that's probably not enough immunity in the population to stop the virus from spreading."

Goodman is now working as a professor at Georgetown University in Washington, D.C.

Vaccinated but infectious

Even an imperfect vaccine could help if it means people get less severe health problems and stay out of the hospital, experts note. People who get flu shots usually get less severe cases of the disease than those who do not get an injection, even when the vaccine is not very effective.

Yet, "there is the risk that even the vaccinated people may unconsciously spread infection," Goodman said.

Some experts call this a "worst-case scenario": a vaccine that works well enough to prevent patients from getting sick, but not well enough to stop them from spreading the virus.

The vaccine could produce an immune response that does not completely block the virus but reduces it enough so that the patient does not experience symptoms of the disease.

However, patients without the usual symptoms can still be infectious. That is a big part of what makes COVID-19, the disease caused by the coronavirus, so hard to control.

The vaccine also may not protect everyone equally. There may be "big, big differences" in how children, adults and the very old respond to a vaccine, said Paul Duprex. He is director of the University of Pittsburgh Center for Vaccine Research.

Flu vaccines usually do not work as well in older people, for example. People with some existing health conditions may respond better or worse than others. Also, there may be differences among different ethnic groups.

"Biology is not black and white," Duprex said. "Biology is a super gray thing."

Vaccine indecision

When we can take face masks off also will depend on how many people get their vaccine injections.

The goal is to get enough people immunized so that the virus has a hard time finding new people to infect. That is a point known as herd immunity.

However, opinion studies show that the number of people willing to get a vaccine as soon it becomes available is dropping. In one study, nearly half of respondents said they would not get the vaccine.

"We may be at a point where we actually don't have the ability to get to that herd immunity," said Daniel Larremore, a University of Colorado computer science professor who models disease transmission. If that happens, he added, "we're going to continue to need other measures," such as masking and social distancing.

Once vaccines become widely available, it still might be wise to keep your mask. Scientists will not immediately know how long the new vaccines offer protection.

"If the trial's been going on for six months, we're going to not really know if the immunity lasts three, five or 10 years," Larremore said.

Scientists also do not know how long the body's defenses remain active after a natural infection with the virus.

Viruses related to COVID-19 cause common colds. Such viruses create immune responses that lessen after a few months to a couple years.

Medical experts say people should expect to keep their masks on at least through most of 2021, while the vaccines become available and scientists learn more about them.

Even then, the virus may not disappear completely. COVID-19 may become a common threat to public health.

"I think if you were a betting person, you would say this probably won't completely go away," Georgetown's Goodman said. "But, in nature, strange things happen."

I'm Ashley Thompson. And I'm Caty Weaver.

Steve Baragona reported this story for VOA News. Ashley Thompson adapted it for VOA Learning English. George Grow was the editor.

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armor - n. a hard covering that protects something (such as a vehicle or an animal)

immunize - v. to give (someone) a vaccine to prevent infection by a disease

unconsciously - adv. not aware of something

scenario - n. a description of what could possibly happen

response - n. something that is done as a reaction to something else

symptom - n. a change in the body or mind which indicates that a disease is present

herd - n. a large group of people

transmission - n. the act or process by which something is spread or passed from one person or thing to another