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Stephen Kissler, a Harvard epidemiologist, expects to gather with his family for the holidays and feels “very confident in having a safe and joyful holiday season.”

His confidence stems from two facts: His family members are vaccinated, and they plan to take at-home rapid antigen tests right before they gather.

“The availability of these rapid tests is something I have been hoping for for months and months,” said Kissler, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health. “Alongside vaccination, they can be one of the most important new elements of our ability to stay safe and socialize with confidence.”

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The rapid tests, Kissler explained, tell you whether you are contagious at the moment you take the test, and yield results in 10 to 15 minutes. They correctly identify when a person is infectious about 98 percent of the time.

“If you have enough virus in your body to be infectious, the test will almost always pick you up as positive,” Kissler said.

And what about the risk of a false positive, that someone might miss Thanksgiving because they wrongly thought they were infected? Earlier this month, the Food and Drug Administration warned of the potential for false-positive results with certain test kits from Australian company Ellume, which recalled the kits in question. The problem stemmed from a manufacturing issue. When there is no manufacturing error, Kissler said, the chance of a false positive is “somewhere between 0 and 2 in 1,000.”

“To me those odds are well worth taking the test,” he said.

The tests are especially valuable for children not yet eligible for vaccination, Kissler said. Children are less likely to have symptoms if they’re infected, but they can still spread the virus.

So if everyone planning to gather at Thanksgiving tests negative with a rapid test that day, especially if everyone is vaccinated, Kissler said, people can put aside their masks and enjoy each other’s smiling faces.

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But there are two big problems. The tests have been selling out and can be hard to find. And they’re not cheap. The most common brands, Abbott’s BinaxNOW and Quidel’s QuickVue, retail for $23.99 for a set of two. You can buy them at pharmacies or online.

President Biden’s announcement last week of a $1 billion federal investment in rapid tests is expected to make them more widely available later this year. Also, last week, the FDA approved a new rapid test, Acon Laboratories’ FlowView. With more tests on the market, prices might come down, Kissler said.

But Dr. Michael Mina, another Harvard epidemiologist who has long advocated for the rapid tests as a public health tool, has said the Biden administration plan will allow for only five or six tests a year per person, which is not enough. And he argues that the federal government should provide them to everyone for free, like the United Kingdom does.

(A White House spokesman noted that the plan includes doubling to 20,000 the number of sites in the federal government’s free pharmacy testing program.)

The rapid tests measure the presence of viral proteins. If you test negative, it’s still possible that you have been infected and can become contagious the next day or the day after. But the rapid test reveals whether you are contagious on the day you took it.

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PCR tests look for genetic material from the virus. They’re available at clinics and pharmacies, and at-home PCR kits enable people to collect samples to send to a lab. But it can days to get a result, and while they accurately tell you whether you are infected with the virus, they don’t reveal whether you are able to spread it to others at any given moment, Mina said.

Infected people are contagious for only a few days during the period when they test positive by PCR, Mina explained last week at the University of California San Francisco Grand Rounds. Rapid tests, he said, “have a very unique ability … to answer the question, ‘Am I infectious now?’”

He described rapid tests as “an answer to moving into this endemic phase of the virus so we can celebrate our holidays together without fear of infecting each other.”

That fear persists this year despite hopes for a brighter holiday season. Two factors dimmed the prospects for anxiety-free celebrations. First, a high percentage of US residents have declined to get vaccinated. And second, the Delta variant is so highly transmissible that even vaccinated people can catch the virus and spread it — although they are unlikely to get very sick from it.

So as you plan your holidays, there are several questions to consider, said Dr. Mary W. Montgomery, an infectious disease physician at Brigham and Women’s Hospital who works in the hospital’s travel clinic.

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First, are you and your family members vaccinated? “For those who are fully vaccinated, I think it is OK to travel,” she said.

Those who are unvaccinated pose a risk to themselves and everyone they come into contact with, including vaccinated people, she said. And while vaccinated people rarely suffer severe illness, they can still get sick enough to miss work for a number of days, Montgomery said.

Families can take the opportunity to encourage unvaccinated relatives to get the shot by asking that only vaccinated people come inside. “That’s a hard thing to say,” she acknowledged.

Another consideration: Who are you traveling with or traveling to see? People who are frail or immunocompromised are less likely to have a robust immune response to the vaccine and are at greater risk if they become infected. They may require extra precautions even if they’ve had booster shots.

“If you are going to visit your 85-year-old mother who is suffering from cancer, you might think about what are the right decisions to make,” Montgomery said. “The decision for every family has to be different.”

You may decide it’s too risky to visit. Or you may choose to wear masks in the presence of vulnerable people, eat outside if weather permits, and open windows to improve ventilation inside.

Both Kissler and Montgomery said it is safe to fly. Airplanes filter and exchange the air frequently, and airlines still require masks.

If you do fly, Montgomery advises taking shorter flights where people are less likely to take their masks off to eat or drink.

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The airport poses some risks, which travelers can mitigate by wearing a mask with at least three layers and a metal insert to hold it in place on their nose. She does not recommend eating indoors at the airport.

But the large space and high ceilings lower the transmission risk in airports.

Trains and buses also have mask requirements, air filtration systems, and opportunities for fresh air to come in at each stop, Montgomery said.

“If there’s a way to drive to a location, that’s preferable,” she said.

“The good news is that with vaccination,” she said, “the likelihood of anyone developing severe disease is very low.”


Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer.