Even those who have had Covid-19 should take precautions to avoid contracting it again, after a patient in the US became more seriously ill the second time, researchers said.
A 25-year-old man from Nevada developed a sore throat, cough, headache, nausea and diarrhoea after catching the virus in late March.
He fully recovered by April 27, but fell ill a second time in late May with similar symptoms that worsened.
He was given emergency oxygen to treat breathing difficulty.
The patient, who was hospitalised during the second infection, did not have any underlying health conditions which put him at risk of developing more serious disease.
Scientists said his case, and others like it, prove it will not be possible to attain herd immunity through natural infection.
"Not only is this strategy lethal for many but also it is not effective," wrote Akiko Iwasaki, an immunologist at the Yale School of Medicine in The Lancet.
“Herd immunity requires safe and effective vaccines and robust vaccination implementation.”
Researchers at the University of Nevada said the US man’s reinfection was confirmed through analysis, which showed he had contracted two variants of the virus.
"Thus, previous exposure to SARS-CoV-2 might not guarantee total immunity in all cases," they wrote in The Lancet.
“All individuals, whether previously diagnosed with Covid-19 or not, should take identical precautions to avoid infection with SARS-CoV-2. The implications of reinfections could be relevant for vaccine development and application.”
There have now been several documented cases of Covid-19 reinfections around the world.
Of the four reinfections cases studied by researchers, symptoms were worse in two patients, suggesting no clear pattern.
Experts have said it could be because they were exposed to a larger dose of the virus the second time around.
But it could also be that the initial immune response worsened the symptoms during reinfection, in a phenomenon known as antibody dependent enhancement.
It was also seen in SARS, a close cousin of the new virus, in experiments.
It is not clear how often reinfections of Covid-19 occur. The phenomenon appears to be rare.
But some experts caution the number of asymptomatic reinfections are likely to be “severely underestimated”.
“Due to the paucity of broad testing and surveillance, we do not know how frequently reinfection occurs among individuals who recovered from their first infection,” wrote professor Iwasaki.
“Asymptomatic reinfection cases can only be picked up by routine community testing or at an airport, for example, and we are probably severely underestimating the number of asymptomatic reinfections.”
She said there is no evidence to suggest second reinfections are as result of "immune evasion" because of variants in the virus.
Studies have shown asymptomatic infections of Covid-19 tend to produce low, and in some cases, even undetectable antibodies, while those who suffer more symptoms mount a stronger immune response.
Higher antibodies appear to last longer. But it is still not known if the antibodies generated are protective.
Experts have said if SARS-Cov-2, which causes Covid-19, follows the same pattern as other coronaviruses, reinfections will become common.
Research has shown it is possible to catch a cold caused by the same strain months later as immunity fades quickly.
In one study from Kenya in 2018, almost 30 per cent of those who caught one variant of a coronavirus experienced a second reinfection. Around 10 per cent caught it a third time and one person was infected four times.
A number of reinfections occurred only three months after the first bout, and in multiple cases the viral load actually increased, "revealing ineffective protective immune responses after initial exposure".
Experts say vaccines generate immune responses in a different way to natural infections, and are expected to be protective.