Can you get Covid-19 twice? Here's what experts say

Experts agree that claims of recurring infections require more study since we are only months into the health crisis and evidence has been anecdotal. 

But if it's proven that recovered patients can 'catch' the virus a second time, it would affect their own immunity while also complicating efforts to obtain the Holy Grail of current medical research - effective vaccines.

Recovery from the disease provides antibodies to fight off the infection. The shelf life of those antibodies, however, may be insufficient to protect a patient for very long or promote long-term immunity across populations.

What have doctors said?

"The possibility of reinfection is certainly real," Dr Robert Glatter, an emergency physician at Lenox Hill Hospital in New York City, told our sister title, USA TODAY. "And one that I am seeing repeatedly on the front lines."

Glatter says he has cared for a "number of patients" who suffer only mild initial infections, get better and actually test negative for the virus before experiencing a recurrence of symptoms. The intensity can be worse the second time, he says. 

"These patients develop difficulty breathing, leading to hypoxia, aches, chest pain, with recurrent and unrelenting fevers and chills," he said, adding that they then test positive again. 

What have experts said?

Experts caution that it's been only a few months since the first surge of cases in China and around the world.

The frequency and severity of reinfections won't be known until cases rise in the areas where there were a significant number of infections last winter, said Dr Daniel Griffin, chief of infectious diseases at ProHEALTH Care in New York.

"We are months away from knowing for certain if reinfections are possible or a significant issue," Dr Griffin told USA TODAY.

Physicians don't know for sure whether people who appear ill a second time are suffering from a resurgence of their first infection, Dr Griffin said.

But he tells the story of a man he treated for Covid-19 as an outpatient in March.

Four months later, he was sick again, this time hospitalised with fevers and chills. He tested positive, the high level of antibodies he had displayed after his illness barely detectable.

Griffin noted that work on other coronaviruses demonstrated people were just as likely to be reinfected a year later as people who were never infected. Studies show Covid-19 antibodies declining in all patients after two months and becoming negative in a high percentage of patients, he said.

 "It's concerning for those of us who hope this virus is just a one-and-done for our patients," Griffin said.

'We have not seen a single convincing case of reinfection'

Dr Monica Gandhi, professor of medicine and infectious disease expert at the University of California, San Francisco, isn't so sure.

“We have not seen a single convincing case of reinfection," she said. "That’s the proof in the pudding that being infected confers immunity."

What have researchers in London found?

In London, King’s College researchers have found that antibodies peaked up to three weeks after onset of symptoms before declining.

The study of 90 patients and health care workers was published in in a preprint paper in medRxiv and has not yet been the subject of scrupulous peer review.

But the research supports the possibility that Covid-19 could reinfect people repeatedly. Blood tests revealed that while 60 per cent of people presented a “potent” antibody response while battling the virus, only 17 per cent obtained that potency three months later. 

All of which could dash the hopes of herd immunity through a one-shot vaccine or community spread of the virus.

What have other studies found?

Melissa Nolan, an infectious disease expert and professor at the University of South Carolina, says other studies indicate that people who were infected but presented no symptoms likely never had sufficient viral loads in their pulmonary tract to generate a sufficient antibody response.

In contrast, people with severe symptoms likely had a higher volume of virus deep in their lungs that "sparked an immunologic chain reaction" resulting in the generation of antibodies. 

Still, one study showed 13 per cent of symptomatic cases had lost immunity and were susceptible to reinfection at two months post-infection.

"My clinical interpretation from these new studies is that until we have a vaccine that is widely available, societies will not naturally develop their own herd immunity," Nolan said. "These new findings suggest that persons might get reinfected."

Ogbonnaya Omenka, a public health expert and assistant professor at Butler University, emphasizes that more research is required.

Natural individual immunity has been shown to be a key factor of vaccine development, he says, so the absence of protection after recovering from Covid-19 could make the vaccine process more complicated. 

Achieving herd immunity would require nearly 75 per cent of the population to be either infected or vaccinated – a goal that may prove to be elusive in light of inconsistent antibody responses and waning levels, Glatter said.

"This raises important issues going forward when we begin vaccinating the population," he said. "It’s likely we will need a booster shot to bolster immunity. We certainly need more data and larger studies to confirm this finding."

Omenka said it remains unclear whether the reinfections represent a common public health phenomenon or they are "outliers" – possibly even victims of inaccurate testing, he said. 

• The original version of this article appeared on our sister website, USA TODAY.