How Contagious Is the New COVID Variant?

Each new COVID variant tends to take over because it spreads better than the one it replaces, and today’s versions are among the most contagious yet. Much of that edge comes from getting past existing immunity, not from becoming a stronger virus.¹ People with past shots or infections can still catch it, but that protection still helps keep most of them out of the hospital.

How the New Variant Compares With Earlier Ones

The original virus spread to about 2.8 new people per case in a population with no immunity.² When each case infects more than one other person, cases grow; when fewer, an outbreak shrinks. Alpha raised the number to roughly 4 to 5, and Delta to about 5, which made Delta roughly 40 to 60% more transmissible than the original.³ Real-world numbers are lower now because so many people have some immunity, and crowding, masks and ventilation change the figure as much as the virus does.

When Omicron arrived in late 2021, its early spread rate in the US was clearly higher than any earlier variant’s.⁴ It drove large waves across US states before its spread slowed in early 2022.⁵

The versions circulating now, with names like LP.8.1.1, XEC, XFG and NB.1.8.1, all descend from Omicron’s JN.1 line.⁶ Most new names are small steps within the JN.1 family, not a brand-new virus, and a new name alone does not signal a big jump in danger.

Estimates of exactly how contagious a variant is differ a lot between studies.⁷ The same variant can look more or less contagious depending on how many people nearby already have immunity, so a number measured in one city or period does not carry over exactly to another. Studies also measure different things: how fast cases are growing, how many people each case infects, or how often household contacts get sick. Those are not interchangeable. The reliable takeaway is the rough ranking, with the original virus lowest, Delta higher and Omicron-family versions higher still.

Why Past Shots and Infections Don’t Stop It

Most of the changes in new variants sit on the part of the spike protein that latches onto human cells.⁸ Some of these changes help the virus grip cells more tightly, and one common change made that grip about 10 times stronger.⁹ The same changes weaken how well antibodies recognize the virus.¹⁰ As a result, blood from vaccinated and previously infected people blocks JN.1 far less well than it blocked earlier Omicron versions,¹¹ and the newest JN.1 offshoots slip past antibodies even more.¹²

Protection also fades with time. Antibody levels after vaccination drop by about 3 months and reach their lowest around 6 months.¹³ Falling antibodies were linked to more breakthrough infections and reinfections, especially with Omicron.¹⁴ More than 6 months since your last shot or infection was linked to a higher reinfection risk.¹⁵ People with both vaccination and past infection kept antibody levels up longer.¹⁶ Protection against infection is strongest in the first few months after a shot, so people at higher risk may time a dose before winter or travel. Updated shots are designed to match recent variants more closely.

Antibodies in the nose and blood are the first barrier that stops infection. When a variant dodges them, you can still catch it and spread it. A second line of defense holds up better: immune cells that clear infected cells still recognize at least 90% of the spike in all Omicron versions tested.¹⁷ They work after infection starts and tend to keep illness milder, so even when antibodies miss, the body can still limit how sick you get. Updated vaccines cut the risk of hospitalization by about half in adults, including those 65 and older.¹⁸

Many people with past shots or infections who catch a current variant have a cold-like illness, with sore throat, congestion, cough or fatigue, rather than a severe one. Immunity does not make you non-contagious, so you should still test, stay away from vulnerable people and take precautions while sick. Anyone with a high-risk condition who tests positive should ask a clinician promptly about treatment, since some antiviral treatments work best when started early.

How Fast It Moves From One Person to the Next

The time from exposure to first symptoms has shortened with each new variant.¹⁹ It averaged about 4.6 days for the original virus and about 3.6 days for Omicron.²⁰ The gap between one person’s symptoms and the next person’s is as short as about 2.4 days with Omicron.²¹ By the time you hear a friend has COVID, you may already be infected.

Symptoms, if they come, often start around day 3 or 4 after exposure, and you may be contagious a day or so before that. That timing is an average, so some people feel ill in a day or two while others take a week, and a shorter Omicron average doesn’t mean you can rule yourself out after three or four days.

Virus levels in the nose peak within about 3 days after symptoms start,²² which makes the first few days of symptoms the most contagious stretch. People who have not yet developed symptoms account for an estimated 40% or more of spread.²³ People who never develop symptoms cause a smaller share, mostly under 15%.²⁴ Staying home only once you feel sick misses part of the contagious window.

After a known exposure, watch for symptoms over the next week, especially days 2 to 5, and wear a mask around vulnerable people during that time. Test when symptoms appear rather than only on day 1.

Your Chances of Catching It at Home

Before Omicron, about 3 in 10 household members caught COVID from a sick relative during Delta.²⁵ In Japan, that share rose from about 25% during Delta to about 32% during Omicron.²⁶ In a Spanish study, more than half of household contacts were infected during Omicron.²⁷

Those percentages describe averages across many households, not the odds for yours. In many homes only the first case gets sick, while in others nearly everyone does, and the average blends these outcomes. Studies also counted infections differently, with some relying on symptoms and others on routine testing, which can raise the reported numbers. Home size, crowding and time spent together differ widely, and so does how recently each person was vaccinated or infected. How quickly the sick person isolates changes the outcome too.

If someone at home is sick, have them use a separate room and bathroom if possible. Open windows or run a fan pointing outward to move air out. Wear a well-fitting mask in shared spaces for the first several days, and avoid eating together, since masks come off at meals.

Why a Home Test Can Miss It Early

Rapid antigen tests picked up only about 47% of infections that a lab PCR test found.²⁸ They did better at the question that matters for spreading: they caught about 80% of people carrying live, spreadable virus.²⁹ Early on, though, they struggle. During Omicron, two popular rapid tests missed nearly every infection in the first 48 hours after a lab test turned positive,³⁰ and some people passed the virus on before their rapid test turned positive.³¹

Rapid tests detect viral proteins and need a fairly large amount of virus in the nose to turn positive, which takes time to build after infection. In the first day or two the level can still be low, so the test is negative even though you are infected. A lab PCR test detects tiny amounts of genetic material, so it can turn positive earlier, but it can also stay positive after you are no longer contagious. A rapid test works better as a check of whether you are contagious than as a way to rule out infection.

If you have symptoms and test negative, test again in 48 hours. Take a third test if the second is negative but you are still unsure, or get a lab PCR test if you need a firmer answer. Keep masking around others until a later test or your symptoms clear.

A positive rapid test is very likely correct, especially if you have symptoms or a known exposure, and means you should act on it. It usually signals enough virus to spread, and most people don’t need a confirmation test.

More Contagious Doesn’t Mean More Severe

Omicron versions multiply readily in the upper airways but are less likely to invade the lungs than Delta.³² Hospitalized Omicron patients had pneumonia less often than patients with earlier variants,³³ and Omicron produced lower peak virus levels than Delta.³⁴ Spreading depends on how readily the virus infects the nose and throat and passes through the air, while severity depends on whether it reaches deep into the lungs and how the body reacts. A mutation can change one without changing the other, which is why Omicron-family versions spread faster than Delta while causing less lung disease on average.

Part of the lower severity is that most people now have some immunity from vaccines or earlier infection. Part may come from the virus itself behaving differently. It is hard to separate the two, so the safe conclusion is that illness is generally milder today, not that the virus is harmless.

Older adults remain the group most often hospitalized, and weakened immune systems and chronic conditions raise risk. Milder on average does not mean mild for everyone, and faster spread means more total cases, so more serious cases overall. One Omicron offshoot, BA.5, regained an older, efficient way of entering cells, a reminder that each new version needs watching.³⁵

What Cuts Your Risk Indoors

Most particles people breathe out are smaller than 1 to 2 micrometers, far finer than a hair.³⁶ They float rather than falling quickly,³⁷ and the virus stays stable in floating particles for more than an hour.³⁸ In a closed room with little air exchange the particles can linger for a long time, and the risk is highest in small, crowded spaces with poor airflow, such as a car with the windows closed or a small meeting room. Ventilation and filtration are what remove these small particles.³⁹ If you must enter a room that someone with COVID has been in, letting it air out first reduces the risk.

In one model of a restaurant, replacing room air 12 times an hour prevented about 54 infections a year.⁴⁰ Open windows on opposite sides for a cross-breeze, or run a portable HEPA purifier sized for the room. With a central system, set the fan to on and use a higher-grade filter. When possible during a wave, meet outdoors.

A well-fitting N95 or KN95 filters fine particles better than cloth. Fit matters as much as material, since gaps let air leak around the edges. Masks are most useful in crowded indoor spaces and around sick people.