Concerns were raised after South Korean health authorities reported more than 300 people who had already beaten the disease tested positive again. Covid-19 patients who recovered but later test positive for the virus are still secreting dead lung cells, not contracting the infection again, WHO told AFP.
If patients who have tackled the disease could possibly contract the virus again, it could pose a problem to efforts to ease the lockdown and find a vaccine.
The remaining lung cells and left over material that causes the “false positives” are likely not infectious.
An expert in virology told MailOnline that once the pathogen is disabled by the immune system and forms a complex bond with an antibody, it stops being infectious but can be identified by a swab test.
“We are aware that some patients test positive after they clinically recover,” a WHO spokesperson said.
“From what we currently know – and this is based on very recent data – it seems they these patients are expelling left over materials from their lungs, as part of the recovery phase.”
For some pathogens such as measles, patients who have overcome it become immune for life.
In the case of coronaviruses such as SARS, the immunity lasted from just months to a couple of years.
People with SARS-CoV-2 – the type of virus that causes COVID-19 – build up antibodies starting a week or so after contracting it or since they first showed symptoms, research has revealed.
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However, it is not apparent whether the body creates enough immunity to fight off a new intrusion by the virus or, if it does, how long such immunity lasts.
“We need systematic collection of samples from recovered patients to better understand how long they shed live virus,” the WHO spokesperson said.
“We also need to understand if this means they can pass the virus to other people – having live virus does not necessarily mean it can be passed to another person.”
WHO said more has to be investigated into the cases of patients who beat the disease but tested positive again days later.
In a recent interview with BBC, infectious disease epidemiologist Maria Van Kerhove, a member of the WHO’s Health Emergencies Program, explained the “dead cell” theory.
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“As the lungs heal, there are parts of the lung that are dead cells that are coming up,” she said, talking on The Andrew Marr Show on Sunday.
“These are fragments of the lungs that are actually testing positive.
“It is not infectious virus, it’s not reinfection, it’s not reactivation – it is actually part of the healing process that is being captured again as being positive.
“Does that mean they have immunity? Does that mean they have a strong protection against reinfection? We don’t know the answer to that yet.”
Ian Jones, Professor of Virology at the University of Reading, told MailOnline that when the virus latched onto an antibody, it becomes unable to infect again.
“However viral genome, the material that is tested by the RT-PCR test, can still be present and can be detected.
“It is probably the reason there have also been reports of virus in blood, semen and faeces.
“In all cases it is likely to the expelled inactivated virus rather than infectious virus.”
In South Korea, officials are still looking for evidence to back their theory that the particles are from “dead” virus cells, KCDC director Jeong Eun-kyeong said.
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