Until recently, there was no pill you could take after a close COVID-19 exposure to lower your chances of getting sick.
Vaccines could help lower your risk of severe illness. Masks, better air flow, and staying away from someone who is sick could help lower your chance of exposure. And antiviral drugs like Paxlovid could help some people once they already had COVID.
But there was a gap: What could you take if someone in your home tested positive and you didn’t have symptoms yet?
In May 2026, the FDA approved ensitrelvir (Xocova) for post-exposure prevention of COVID-19 in adults and young people 12 and older after contact with someone who has COVID. It’s the first oral antiviral approved in the U.S. for this use.
Ensitrelvir isn’t a vaccine. It also isn’t approved in the U.S. to treat COVID once you’re sick. It’s meant to be taken early, after exposure, to help lower the chance that you’ll develop symptomatic COVID.
You may be wondering how a drug can do that, how well it worked in studies, and who should ask about it. Here’s how ensitrelvir went from antiviral research to FDA approval.
Step 1: The Science Behind Ensitrelvir (Xocova)
To understand how ensitrelvir works, it helps to know what viruses do once they get into the body.
A virus can’t make copies of itself on its own. It has to get inside your cells and use them to multiply. SARS-CoV-2, the virus that causes COVID-19, also makes a key enzyme called the main protease, or Mpro. The virus needs this enzyme to make more copies of itself.
Ensitrelvir blocks that enzyme. That’s why it’s called a protease inhibitor.
Justin Moore, PharmD, an infectious diseases clinical pharmacist at Northwestern Memorial Hospital in Chicago, Illinois, says ensitrelvir works like other antivirals in a basic way: If you take it early, it gets in the virus’ way before the virus can build up enough to cause symptoms.
The timing part is important. If the virus has already copied itself many times, it’s harder for a drug to change what happens next. But if you take the drug very early after exposure — usually within a few days — there may be a chance to slow the virus before you feel sick.
How is ensitrelvir different from a vaccine?
Vaccines teach your immune system how to spot a germ. This helps your body respond faster and better if you’re exposed to the virus later.
An antiviral, such as ensitrelvir, protects you in a different way. Instead of training your immune system to fight a foreign invader, it blocks a step the virus needs to make copies of itself.
But just because an antiviral might stop you from getting symptoms of COVID after an exposure doesn’t mean it replaces vaccination.
Geeta Sood, MD, assistant professor of medicine at Johns Hopkins University School of Medicine, says vaccines are still a first-line tool for COVID prevention. Ensitrelvir adds another layer of protection after a known exposure.
Sood and Moore stress that a multi-step approach to COVID prevention is best. In real life, that might look like this: You’re up to date on vaccines, but someone in your house tests positive. You don’t have symptoms yet. Ensitrelvir gives you an extra way to lower your odds of getting sick.
Step 2: Early Testing
Before a drugmaker sends a medication for FDA approval, researchers first need to know if the idea will work.
Ensitrelvir was developed through research on SARS-CoV-2 protease inhibitors. Lab studies looked at whether the drug could block the virus’s main protease, including against newer variants. Early lab and animal studies helped show that the drug could safely stop viral replication in controlled settings.
After these promising results, researchers moved into human studies.
Step 3: Testing in People
Clinical trials are studies that test a drug in people. Each phase is designed to answer a different question before moving forward.
One important note: Early studies of ensitrelvir mostly looked at the drug as a treatment for people who already had COVID or SARS-CoV-2 infection. Those studies helped researchers understand the dose, safety, and antiviral effect. Later trials looked at whether ensitrelvir could help prevent symptomatic COVID after exposure.
Phase 1: Is it safe? How does it move through the body?
These earliest human studies of ensitrelvir were phase 1 trials in healthy adults. These studies weren’t meant to prove whether the drug could prevent COVID. Instead, researchers wanted to know how the drug acted in the body and whether it caused side effects or other health issues.
Researchers tested single and multiple doses of ensitrelvir. They watched how quickly the drug was absorbed, how long it stayed in the body, and whether food changed the way the drug worked.
The drug stayed in the body long enough to support once-a-day dosing. That mattered because a simpler schedule can make a drug easier to take correctly.
Phase 2: Is it safe? Does it work? What dose should be studied next?
These studies included people with mild to moderate COVID or had tested positive for SARS-CoV-2 but didn’t have symptoms. The drug helped lower the amount of virus in the body quickly. It was also generally well tolerated, and side effects were mild to moderate.
Those results helped researchers ask a new question: If ensitrelvir can lower the amount of virus after infection, could it also help stop people from getting sick if they take it very early after exposure?
Phase 3: Larger trials with more people
These larger studies are usually the last major step before a drug company asks the FDA to review a medicine.
Ensitrelvir had already been studied in a phase 3 treatment trial called SCORPIO-SR. This study included more than 1,800 people with mild to moderate COVID in Japan, Vietnam, and South Korea. People took ensitrelvir or placebo once a day for five days.
In that study, ensitrelvir shortened the time it took for common COVID symptoms to go away by about a day. People started treatment within 72 hours of symptom onset. The drug also lowered viral levels. No new safety concerns showed up.
Japan gave ensitrelvir emergency approval in 2022 and full approval in 2024 for mild to moderate COVID. For U.S. approval, the key study was a different phase 3 trial called SCORPIO-PEP. PEP stands for post-exposure prophylaxis.
This trial tested whether ensitrelvir could help prevent COVID after someone had been exposed at home.
Who was in the SCORPIO-PEP study?
SCORPIO-PEP included people 12 and older who lived with someone who had COVID.
To join the study, household contacts had to test negative for SARS-CoV2 and have no COVID symptoms when they enrolled. Treatment had to start within 72 hours of when the infected household member first had symptoms.
Researchers tested the drug during this three-day window because antivirals work best when used early, before the virus has a chance to make lots of copies of itself.
How did the study work?
The trial was randomized, double-blind, and placebo-controlled.
That means people were randomly assigned to get ensitrelvir or a placebo. (A placebo is a lookalike drug that has no medicine.) During the trial, neither the people in the study nor the researchers knew who got which one.
This kind of study is considered strong because it helps reduce bias.
People took ensitrelvir once a day for five days. The dose was higher on the first day, which is called a loading dose. They took a lower dose on days two through five.
Researchers then watched to see who got symptomatic COVID.
What did the results show?
About 3% of people who took ensitrelvir developed symptomatic COVID by day 10. That compared with about 9% of people who took the placebo.
That worked out to about 67% lower relative risk of getting COVID symptoms. The drug also showed a 34% relative risk reduction in passing SARS-CoV-2 to someone else in the household.
In simple terms, that means the drug didn’t prevent every infection. But it did make symptomatic COVID less likely after household exposure.
Sood said the results showed a “pretty dramatic and remarkable impact,” especially because people were tested carefully with PCR testing and symptom checks.
Moore says this is what sets ensitrelvir apart. Other COVID antivirals have been used to lower the risk of severe outcomes, like needing treatment in the hospital or dying from an infection. Ensitrelvir is different because the goal is to stop you from getting symptoms in the first place.
What about long COVID?
The main goal of SCORPIO-PEP was to see if ensitrelvir could prevent symptomatic COVID in household members after exposure.
Researchers also looked at longer-term symptoms, though the study wasn’t mainly designed to prove whether the drug prevents long COVID. But Moore says it’s reasonable to think that if you don’t get COVID, your risk of long-term symptoms from an infection would go down.
What about side effects?
Side effects were similar between the ensitrelvir and placebo groups. The most common ones were mild and went away on their own. They included things like headache, diarrhea, and cough.
Earlier studies also found that ensitrelvir could cause short-term changes in the amount of fat in your blood, including lower HDL cholesterol and higher triglycerides.
Ensitrelvir can interact with other medicines. It doesn’t have ritonavir, the boosting drug in Paxlovid that’s known for many drug interactions. But your doctor or pharmacist should review your medication list before you take it.
Some people have had serious allergic reactions to ensitrelvir, though this appears to be rare. Call your doctor or get emergency help right away if you take the drug and have trouble breathing, swelling in your face or throat, hives, or serious dizziness.
Step 4: Checking by Experts
Clinical trial results don’t mean a drug goes straight to pharmacy shelves. First, they go through FDA review. That review is handled by the FDA’s Center for Drug Evaluation and Research, or CDER.
This is the group that checks whether a new medicine works, whether it’s safe enough for the people who may use it, and whether the pros outweigh the cons.
For ensitrelvir, CDER reviewed the SCORPIO-PEP results, including the study design, who took part, and how many people developed COVID symptoms after being around a sick household member. Reviewers also looked at side effects and earlier safety data from studies of ensitrelvir in people with mild to moderate COVID.
In the end, the FDA agreed the evidence was strong enough to approve ensitrelvir for post-exposure prevention of COVID-19 in adults and children 12 and older.
Step 5: Watching Over Time
Even if a drug works well in a study, doctors and researchers keep an eye on long-term safety and effectiveness.
Real-world use can answer questions that may not show up in clinical trials. For example: Who benefits most? Can people get the drug fast enough for it to work? Will insurance cover it? Are there unexpected side effects or drug interactions? Does it still work if the virus changes?
When Should You Ask About Ensitrelvir?
Ensitrelvir should be taken as soon as possible and within 72 hours after you’re around someone who has COVID. But that may take some planning.
If you’re interested in learning more, Moore says to consider discussing ensitrelvir with your doctor before you get sick. It may be especially important to ask about fast access to the drug if you:
- Have a weakened immune system
- Are an older adult
- Live with someone at higher risk
- Have chronic conditions like kidney, liver, or lung disease
- Have had a hard recovery from COVID before
- Haven’t gotten vaccinated for COVID recently
Ensitrelvir may not be right for everyone. It depends on your age, health history, other medicines, timing of exposure, cost, whether you’re pregnant or breastfeeding, and access. Your doctor will let you know what’s the best choice for your situation.
