If you still haven’t had COVID, are you immune — or just lucky?

The estimated percentage of people who have contracted the coronavirus ranges from 70% to 90% of the US population, but it’s unclear how many are truly uninfected as asymptomatic infections and home testing have muddyed the waters.

Experts say the Bay Area is likely to have a higher rate of COVID super-dodgers than other major metropolitan areas as a higher proportion of residents are vaccinated and stimulant, and much of the population has adopted mitigating factors such as masking or social distancing take during the pandemic.

So what’s up with this shrinking subset of COVID holdouts? How have they avoided the coronavirus for so long and is it inevitable that they will eventually become infected?

Here’s what the experts say, as well as several Bay Area residents who shared their stories.

Prevent infection

There are several reasons why some people have remained COVID-free, experts say. They included vaccination status, masking, the type of variant circulating, lifestyle choices that lowered overall risk – and just plain luck.

“It’s certainly hard to imagine that someone who hasn’t been vaccinated, or hasn’t been careful, especially with masks, couldn’t stay infected,” said Dr. Bob Wachter, chief of medicine at UCSF. “But these are all risk factors.”

Wachter has detailed his personal COVID strategy throughout the pandemic on his popular Twitter account and has not yet tested positive, even though his wife contracted the coronavirus in May 2022.

He has been careful, he said, but luck also played a role.

“I’ve had five vaccinations, don’t live with small children, have been relatively careful about masking, and I still attribute the fact that I haven’t had COVID partly to luck,” he said, adding that there are plenty of people who have received all their recommended injections and have been careful, but have nevertheless become infected.

Socioeconomic status may also play a role, said UC Berkeley infectious disease expert and emeritus professor Dr. John Swartzberg, disproportionately affecting low-income and minority communities.

“We know there is a direct correlation between your income and the likelihood of becoming infected, as well as hospitalization and death,” he said.

The transmissibility of the variant also determines the infection rate, Swartzberg said, as omicron and its subvariants are much more transmissible than their predecessors.

Asymptomatic infection and immunity

Wachter has taken coronavirus tests whenever he had symptoms, always with a negative result.

“But asymptomatic infection is so common that I think there’s a reasonable chance I’ve had asymptomatic COVID,” he said. “The fact that my wife had it doesn’t change the odds. We know that the household attack rate is less than 50%, so a lot of people are in my situation: a family member had it, but they didn’t.

There is also the possibility that some of the population has natural immunity to the coronavirus.

Early evidence suggested people with type O blood might be better protected, but Wachter said the theory hasn’t panned out in subsequent studies.

Instead, some individuals may have mutations in their genes that make them resistant to infection with the virus, experts say, just as there are individuals who are naturally “immune,” or resistant to HIV and the plague.

UCSF infectious disease expert Dr. Peter Chin-Hong said for COVID that these mutations may involve different receptors:

• ACE2: Where the coronavirus enters the body and where a mutation in that receptor makes it difficult for the virus to spread.

• Enzymes: Chemicals that speed up reactions in cells.

• immune cells: Antibodies or T and B cells.

But it’s still unclear how many people have this natural immunity, and Swartzberg said a better understanding of how COVID infects our cells is needed first.

“I suspect it will be a very small percentage of the population” that is naturally immune to the virus, he said. “People should not assume that they cannot get infected. That could be a dangerous mistake.”

Some of the lucky few in the Bay Area

Bay area residents contacted by The Chronicle who said they have not tested positive for the coronavirus are diverging in their pandemic approach: some remain very cautious, some have fully resumed their pre-pandemic lives, and some are in between .

My husband and I are probably in the ‘in-between’ category now – but until recently we were rather cautious about COVID due to personal reasons and have yet to test positive. We received all available vaccines and boosters, worked from home for the most part, wore high-quality masks in indoor public areas, only ate out at restaurants and, aside from our wedding last spring, avoided large events and gatherings. I have not been on a plane since January 2020.

Susan Taylor, 65, lives in San Francisco and still wears a mask during transit and in crowded indoor environments. She and her partner follow Wachter’s tweets, which she calls invaluable, and will only eat out during COVID peaks, then move indoors when transmission rates are low.

“I test when I feel like I have a cold,” Taylor said. “So far, so good. I’m retired, so I don’t have to go back to an office environment.”

Taylor said she has “traveled a lot”, including to Europe, attending gigs and the occasional Warriors game.

“I’m mostly staying cautious because I really don’t want to catch COVID and because I feel like I’ve learned this helpful, easy way to stay healthier,” she said. “Wearing a mask is simple and easy, and I’ve hardly had any colds or other illnesses, which is a huge bonus.”

Stefanie Lingel, 47, who lives in Los Gatos with her partner, has also yet to test positive for the coronavirus — although she was in New York City at the start of the pandemic, she said, and had to go to the hospital for an injury just when the emergency room filled up with COVID patients.

She avoided contracting the virus again when her partner tested positive for the virus last June.

“We live in a very small studio loft,” she said. “He was banished to the loft and I stayed downstairs.” If one of them came down or went up, they would mask themselves and sleep separately.

“We were both testing and I never got it,” she said.

Lingel has been vaccinated and had one booster shot, and has maintained COVID measures throughout the pandemic, such as social distancing and masking in indoor areas, including when going to the office two days a week.

If she’s feeling a little unwell, she said, she’ll do a COVID test, but they’ve all come back negative. She added that she “gets sick very rarely” and has no lingering health problems, and she wonders if that helped her avoid infection.

Alameda resident Aaron Rubin, 55, lives with his wife, and his teenage son was with them during the pandemic and only recently left for college. Rubin, a lawyer, worked remotely during the lockdown and now goes to the office once or twice a week. He is fully vaccinated and stimulated, and has been following health protocols, loosening up as they have been lifted.

“From the beginning, my attitude was that I would just do what the health authorities recommend,” he said. “As long as there were mandates, I followed them and did what was expected of me.”

Once the mask mandates ended, he “pretty much stopped wearing a mask” unless he’s with someone who is careful, he said.

“I’ve been to concerts, I’ve been on planes, I’ve been in crowds at South by Southwest,” he said, referring to the music festival in Austin, Texas.

He said his family has never tested positive for the coronavirus at home, even if they have had symptoms. They didn’t get PCR testing because Rubin said the symptoms were never severe.

“It could have been a cold, it could have been the flu, it could have been COVID, I have no idea,” he said, acknowledging that home tests are not 100 percent reliable and that he could have had a false negative.

Scott Goodman, 34, lives in San Francisco with two roommates. He has had all available vaccine and booster shots and said he will stop consistent indoor masking by the end of 2021. He only continues to mask on airplanes and sometimes during transportation, and wherever mask wearing is required.

“I’ve gone to a lot of live music, festivals and other public events without much worry,” he said, adding that he had been dining indoors “for a while.”

He thinks it helped to get vaccinated as soon as possible and “go back to vaccinations every six months since then,” as well as living in a city with “very high vaccination rates, no other health issues… and just plain luck.”

“I suppose I could have had it and been completely asymptomatic, but I really couldn’t know that,” he said.

Is COVID inevitable?

While a small percentage of people have managed to stay COVID-free, Swartzberg said the coronavirus will be circulating for a long time and reinfections are and will remain common.

And the ranks of the never-infected will continue to shrink, Swartzberg said.

“Given how transmissible omicron and its subvariants are, it’s unlikely that humans will escape to become infected,” he said. “How long can we live a secluded life?”

If you’re up to date on COVID vaccines and know how to access early antiviral therapies like Paxlovid, then “it’s safe to relax restrictions,” Chin-Hong said.

“COVID will be a part of our lives for many years, if not indefinitely,” he said. “It’s important to use all the tools we have and have confidence in them…Embrace life with enthusiasm, but responsibly.”

For Wachter, he said if he had unknowingly had COVID recently, he would “feel a little bit safer” with that extra immunity.

But since six months have passed since he got his bivalent booster, he said his protection against infection has decreased, so he’s a little more cautious. He will choose to dine alfresco whenever possible and take off his mask at a party, but keep it up in large crowds, on planes and in theaters.

“Since COVID will be around forever, I’m taking a little more risk than I used to,” he said. “I think it’s more likely than not that I’ll ever get COVID.”

Reach Kellie Hwang: kellie.hwang@sfchronicle.com; Twitter: @KellieHwang