Within a day of testing positive for covid-19 in June, Miranda Kelly was sick enough to be scared. At 44, with diabetes and high blood pressure, Kelly, a certified nursing assistant, was having trouble breathing, symptoms serious enough to send her to the emergency room.

When her husband, Joe, 46, fell ill with the virus, too, she really got worried, especially about their five teenagers at home: “I thought, ‘I hope to God we don’t wind up on ventilators. We have children. Who’s going to raise these kids?”

But the Kellys, who live in Seattle, had agreed just after their diagnoses to join a clinical trial at the nearby Fred Hutch cancer research center that’s part of an international effort to test an antiviral treatment that could halt covid early in its course.

By the next day, the couple were taking four pills, twice a day. Though they weren’t told whether they had received an active medication or placebo, within a week, they said, their symptoms were better. Within two weeks, they had recovered.

“I don’t know if we got the treatment, but I kind of feel like we did,” Miranda Kelly said. “To have all these underlying conditions, I felt like the recovery was very quick.”

The Kellys have a role in developing what could be the world’s next chance to thwart covid: a short-term regimen of daily pills that can fight the virus early after diagnosis and conceivably prevent symptoms from developing after exposure.

“Oral antivirals have the potential to not only curtail the duration of one’s covid-19 syndrome, but also have the potential to limit transmission to people in your household if you are sick,” said Timothy Sheahan, a virologist at the University of North Carolina-Chapel Hill who has helped pioneer these therapies.

Antivirals are already essential treatments for other viral infections, including hepatitis C and HIV. One of the best known is Tamiflu, the widely prescribed pill that can shorten the duration of influenza and reduce the risk of hospitalization if given quickly.

The medications, developed to treat and prevent viral infections in people and animals, work differently depending on the type. But they can be engineered to boost the immune system to fight infection, block receptors so viruses can’t enter healthy cells, or lower the amount of active virus in the body.

At least three promising antivirals for covid are being tested in clinical trials, with results expected as soon as late fall or winter, said Carl Dieffenbach, director of the Division of AIDS at the National Institute of Allergy and Infectious Diseases, who is overseeing antiviral development.

“I think that we will have answers as to what these pills are capable of within the next several months,” Dieffenbach said.

The top contender is a medication from Merck & Co. and Ridgeback Biotherapeutics called molnupiravir, Dieffenbach said. This is the product being tested in the Kellys’ Seattle trial. Two others include a candidate from Pfizer, known as PF-07321332, and AT-527, an antiviral produced by Roche and Atea Pharmaceuticals.

They work by interfering with the virus’s ability to replicate in human cells. In the case of molnupiravir, the enzyme that copies the viral genetic material is forced to make so many mistakes that the virus can’t reproduce. That, in turn, reduces the patient’s viral load, shortening infection time and preventing the kind of dangerous immune response that can cause serious illness or death.

So far, only one antiviral drug, remdesivir, has been approved to treat covid. But it is given intravenously to patients ill enough to be hospitalized, and is not intended for early, widespread use. By contrast, the top contenders under study can be packaged as pills.

Clinical trials have followed, including an early trial of 202 participants last spring that showed that molnupiravir rapidly reduced the levels of infectious virus. Merck chief executive Robert Davis said this month that the company expects data from its larger phase 3 trials in the coming weeks, with the potential to seek emergency use authorization from the Food and Drug Administration “before year-end.”

Pfizer launched a combined phase 2 and 3 trial of its product Sept. 1, and Atea officials said they expect results from phase 2 and phase 3 trials later this year.