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The arrival of the first COVID-19 vaccine will not mean people can throw their masks away, experts say.
Face coverings, social distancing and rigorous hand-washing still will be required for some time. How long depends on factors including how good the vaccines are and how long protection lasts - questions that will not be answered when the first shots arrive.
For starters, a vaccine may not be 100% effective.
"This vaccine is not likely to be a suit of armor," said Vanderbilt University Medical Center infectious disease professor William Schaffner.
Seasonal influenza vaccines, for example, have ranged from 60% effective at best to 10% at worst in recent years.
The U.S. Food and Drug Administration has set the bar for COVID-19 vaccine efficacy at 50%.
A partially effective vaccine might help high-risk groups such as front-line health workers or those with underlying health problems, but it might not be right for the general population, said former U.S. Food and Drug Administration Chief Scientist Jesse Goodman, an infectious disease professor at Georgetown University.
"Certainly 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."
Vaccinated but infectious
On the plus side, even a less-than-perfect vaccine could help if it means people get less severe illnesses and stay out of the hospital, experts noted. People who get flu shots tend to get milder cases than the unvaccinated, even when the vaccine is not very effective.
On the other hand, "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 tamps it down enough so that the patient does not develop symptoms.
However, asymptomatic patients can still be infectious. That is one of the factors that makes COVID-19, the disease caused by the coronavirus, so hard to control.
Also, the vaccine may not protect everyone equally.
There may be "big, big differences" in how children, adolescents, adults and seniors respond to a vaccine, said University of Pittsburgh Center for Vaccine Research Director Paul Duprex. Influenza vaccines tend to not work as well in older people, for example.
People with some underlying 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 hesitancy
When we can take our masks off also will depend on how many people get their shots.
The goal is to get enough people immunized so that the virus has a hard time finding new people to infect, a point known as herd immunity.
However, polls show declining numbers of people are eager to get a vaccine as soon as it is available.
In one survey, 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, "in which case we're going to continue to need other measures," such as masks and social distancing.
Once the vaccines roll out, it still might be wise to hold onto your mask. Scientists will not know how long these brand-new vaccines give 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 do not know how long the body's defenses remain active after a natural infection with this virus, either.
Viruses related to the COVID-19 coronavirus cause common colds. These viruses generate immune responses that wane after a few months to a couple years.
"What you could assume is, if the virus doesn't give you long-lived immune responses, then potentially the vaccine won't give long-lived immune responses," said Duprex, of the University of Pittsburgh.
Expect to keep your mask on at least through much of 2021 while the vaccines roll out and scientists learn more about them.
Even then, the virus may not disappear completely. COVID-19 may become a regular public health threat.
"I think that if you were a betting person, you would say this probably won't completely go away," Goodman said. "But, in nature, strange things happen."