WASHINGTON (BLOOMBERG) - When Mr Eric Grunor contracted Covid-19 in January, he became so ill that he struggled to get off the couch. One night, he awoke at 3am, winded, barely able to talk and so fatigued he could hardly lift his head.
"I woke my wife up," he said, telling her to take him to the emergency department. "My wife thought she was going to be a widow."
After three weeks of recovery at home, the 54-year-old Texas insurance broker's experience is one few would want to endure twice. But he remains unvaccinated, putting him among a stubborn contingent of Americans who say they have natural immunity and do not require shots - a belief that experts are divided on.
"I'm in the category of persons who would least need the vaccine, at this point," said Mr Grunor. "To me, natural antibodies are better than any man-made antibodies."
He is concerned that inoculated people can still get infected and believes there is a lack of clarity about the shots' long-term safety. Even if he had not been ill, he said, he probably would have remained unvaccinated. His wife and son, who appear to have avoided Covid-19, are not vaccinated either.
More than 100 million people in the United States have likely been infected with Covid-19, according to one recent estimate. Many of them have become proponents of natural immunity who are among the roughly 126 million Americans who remain unvaccinated, about 38 per cent of the population.
As public health officials urge universal immunisation, polling shows more resistance to shots among people with prior infections. The majority report that having had Covid-19 influenced their decision to remain unvaccinated.
The debate over natural immunity fuels hesitancy and foreshadows more challenges for vaccination campaigns as the highly contagious Delta variant rages.
Indeed, some research indicates that an earlier case of Covid-19 protects as well or better against the strain than vaccination alone.
A recent analysis from Israel found that fully inoculated people were at six times higher risk of Covid-19 than those who were previously infected and unvaccinated. Those with earlier infections were also less likely to fall ill with symptoms and get hospitalised with Covid-19, according to the study, published before review by experts in the field.
But other findings suggest different conclusions. A large British study, also published before peer review, found two-dose vaccines were at least as effective as natural immunity.
Last month, a study from the Centres for Disease Control and Prevention found people who caught Covid-19 and remained unvaccinated were more than twice as likely to be reinfected as those who got immunised.
Experts also point to continuing questions about how much protection is given by previous illness, how long it lasts and how well it stacks up against vaccination. Several studies, including the one from Israel, show that at least one shot adds to the protection granted by an infection.
"If you've been naturally infected and you get vaccinated, you are better off," said Mr Paul Offit, director of the Vaccine Education Centre at the Children's Hospital of Philadelphia. "And there's no downside. There is no good reason to not get the vaccine."
The combination of natural immunity and vaccine-induced protection has long been seen as a route out of the pandemic.
In the US, the concept became politicised early on, as opponents of masking, social distancing and other public health measures claimed that when enough people had been infected with the coronavirus, the pandemic would burn out.
Antibodies and other components of the immune system remember invaders, helping protect against further infection. But immunity is complicated and not without limitations. Many infectious diseases, like smallpox, persisted for millennia despite humans' natural immune response, and have been controlled only by vaccination.
The immune system can also be fooled by viral tricks, such as mutation. Sars-CoV-2's interaction with the body still has many mysteries that remain uncracked.
"We don't have any formula where we know what gives you reasonable protective immunity," said Dr Shane Crotty, a professor at La Jolla Institute for Immunology. "We can't look at a single person and say, 'Yes, you are protected'."
After illness with Covid-19, the vast majority of people appear to have significant amounts of immunological memory as measured by antibodies and immune cells, Dr Crotty noted. But about 5 per cent do not, and levels of protection vary dramatically between people.
Then there is the issue of documenting infection. Some people who believe they had Covid-19 were never diagnosed. Even for those who were infected, "what... test do you trust?" Dr Crotty said. "What antibody test do you trust, if you're going to formalise this?"
Meanwhile, vaccines are plentiful in the US and carry only rare serious safety issues, said Mr Brian Castrucci, president and chief executive officer of the de Beaumont Foundation, which has a public health focus. The risks of Covid-19 infection are far greater, he added.
"The path to getting through the pandemic is vaccination," he said. Most people who claim they are protected by natural immunity are "not virologists, they're not public health professionals".
Dr Todd Zywicki, a George Mason University (GMU) law professor, sued school officials in early August over its vaccination policy and other Covid-19 measures, saying that he should not have to comply because he already had had an infection.
The New Civil Liberties Alliance, which represented him, later said Dr Zywicki had been given a medical exemption. But the university does not consider natural immunity a substitute for vaccination, the group said in statement, calling inoculation "medically unnecessary" for those people.
"At times GMU officials have appeared to deny that such a thing as naturally acquired immunity exists," the statement added.
Natural immunity comes up often at the North Carolina hospital where physician Dona Hasou works. Patients as well as colleagues say they cannot get Covid-19 again, or that if they do, they will not get as sick. Many hospital staffers previously contracted the illness, and less than half of them have been immunised, she said.
"'I've had it already, so I'm safe' is part of the reassurance for people," she added.
Often, though, the same people cite fear about the vaccine's side effects.
Many proponents of natural immunity refer to an unreviewed study of employees at the Cleveland Clinic that concluded previously infected people are unlikely to benefit from Covid-19 vaccination.
The medical centre later released statements walking the findings back, pointing out it was conducted prior to the Delta variant's ascendance. Still, some medical providers are also asserting that natural immunity is superior.
"The doctor said there's no reason for me to get a vaccination before my antibody numbers become almost non-existent," said Mr Reed Storey, a 35-year-old farmer in Arkansas, speaking over the whirring of a cotton picker in the background. He declined to name his physician.
Mr Storey said his girlfriend, a nurse who became sick around the same time he did and also has antibodies, also has not got shots.
For public-health outreach workers, it adds up to a lot of frustration. In a focus group of vaccine-hesitant people, Mr Castrucci recounted, a woman said her father died from Covid-19, likely contracted from an unvaccinated healthcare worker.
"And she didn't want to get the vaccine," he said. "I can't make sense of that. I think that is the hardest part here."