According to three new reports, coronavirus immunity can last for months or longer.
The findings suggest that, some people who recover from coronavirus infections stay protected for at least a certain time, reports CNN.
They also suggest that coronavirus vaccines may be able to protect people for more than just a few weeks.
One study found that people produce antibodies that protect against infection and last for at least five to seven months.
"We have one person that is seven months out. We have a handful of people that are five to seven months out," Deepta Bhattacharya, an immunobiologist at the University of Arizona College of Medicine, told CNN.
"We conclude that neutralizing antibodies are stably produced for at least 5-7 months after SARS-CoV-2 infection," his team wrote in a report published in the journal Immunity on Tuesday.
They have been working with county officials to test volunteers in Arizona since April 30, ever since they developed a blood test for coronavirus.
Like many researchers, they found antibodies to the coronavirus spiked immediately following infection and then crashed. But that wasn't the end of the story.
The B cells that create antibodies mature into what are called plasma cells, Bhattacharya said. "Generally, you get a ton of short-lived plasma cells," he said.
"They make a ton of antibodies." But these are not antibodies that do much to protect the body from the virus.
"The best cells compete with each other," he said. "Only those go on to become long-lived cells. Those form later in the response."
The team has tested close to 30,000 people and has looked at some who have been tested several times.
"I think it's good news," Bhattacharya said.
The novel coronavirus has only been around for less than a year, so it will take time to know just how long immunity lasts. "That said, we know that people who were infected with the first SARS coronavirus, which is the most similar virus to SARS-CoV-2, are still seeing immunity 17 years after infection. If SARS-CoV-2 is anything like the first one, we expect antibodies to last at least two years, and it would be unlikely for anything much shorter," he said.
The severe acute respiratory syndrome virus infected close to 8,000 people and killed about 800 before it was stopped in 2004.
People who were sicker with Covid-19 had a stronger immune response, Bhattacharya said. "The people sampled from the ICU had higher levels of antibodies than people who had milder disease." He doesn't yet know what that will mean for long-term immunity.
Plus, the researchers have not checked to see if people were exposed to the virus a second time and were able to resist becoming infected again.
And the studies do not support the idea that the US or any other country could reach herd immunity soon through natural infection. The World Health Organization estimates only 10% of the population has been infected with Covid-19. That leaves a long way to go to herd immunity.
However, two other studies support the idea of long-lasting immunity.
Researchers at Massachusetts General Hospital tested 343 coronavirus patients, most of them severely ill in hospital. They had elevated levels of certain antibodies called IgG antibodies for up to four months, they reported in the journal Science Immunology last week.
"We showed that key antibody responses to Covid-19 do persist," infectious disease specialist Dr. Richelle Charles of Massachusetts General Hospital said in a statement.
Two other antibody types -- IgM and IgA -- first spiked and then crashed in these patients. "We can say now that if a patient has IgA and IgM responses, they were likely infected with the virus within the last two months," Charles said.
"Knowing how long antibody responses last is essential before we can use antibody testing to track the spread of Covid-19 and identify 'hot spots' of the disease," said Dr. Jason Harris, a pediatric infectious disease specialist who worked on the study.
A Canadian team used saliva tests and had similar findings. Their patients had IgG antibodies that lasted up to 115 days after they first developed symptoms.
"This study confirms that serum and saliva IgG antibodies to SARS-CoV-2 are maintained in the majority of Covid-19 patients for at least 3 months post symptom onset," they wrote in Science Immunology.
"This study suggests that if a vaccine is properly designed, it has the potential to induce a durable antibody response that can help protect the vaccinated person against the virus that causes Covid-19," Jennifer Gommerman, professor of immunology at the University of Toronto, said in a statement.
"Our study suggests saliva may serve as an alternative for antibody testing. While saliva is not as sensitive as serum, it is easy to collect," Gommerman added.