Our immunity to COVID-19 may not last more than a couple of months, according to a study.
The immune response of mild COVID-19 patients may be transient—like that gained when we catch common colds, scientists have said a paper submitted to the pre-print server medRxiv. Pre-print means the study has not been through the rigorous peer review process required to publish in scientific journals. Scientists release papers in this way this to spark debate on a topic, and it is particularly useful during fast-moving situation like a pandemic.
Antibodies to SARS-CoV-2, the virus that causes COVID-19, can usually be detected about 10 to 15 days after a person first shows symptoms, the authors said citing existing research. But as the virus is new, it is not clear how long these antibodies last or whether they protect against re-infection, they said.
The team examined blood samples taken periodically from 65 people who tested positive for coronavirus up to 94 days after they first had symptoms. The severity of their illnesses ranged from asymptomatic to serious enough to require life support.
More than 95 percent of the patients had coronavirus antibodies in their blood. After eight days on average, they carried antibodies capable of neutralizing the virus, or nAbs.
nAb levels peaked after 23 days on average, and dropped two to 23-fold in the 18 to 65-day follow-up period. In samples collected after 65 days, the percentage of participants who still had potent nAbs dropped to 16.7 percent.
The extent of the nAb response depended on how sick the person had been, the researchers found. Those who had relatively high levels of nAbs at their peak maintained them after more than 60 days, but those with lower levels returned almost to their baseline level.
In patients with lower nAb levels, they became undetectable or also reached baseline level after about 50 days “highlighting the transient nature of the [antibody] response towards SARS-CoV-2 in some individuals,” the authors wrote.
The researchers also looked at blood samples from 31 healthcare workers at a group of hospitals in the U.K. capital of London who had coronavirus antibodies, and found their nAb levels also dropped over time.
The reason for the link between sickness and antibody levels was unclear, according to the researchers. It may be that having more of the virus may make people more sick and trigger a stronger antibody response. It is also possible that antibodies themselves could make a person sicker.
“This study has important implications when considering protection against re-infection with SARS-CoV-2 and the durability of vaccine protection,” the researchers said.
Experts who did not work on the paper praised the team for their study, but highlighted it had not yet been peer reviewed.
Lawrence Young, Professor of Molecular Oncology at the U.K.’s University of Warwick, said in a statement the analysis was “well-performed” and “provides a thorough examination of different antibody responses to SARS-CoV-2.”
However, future studies involving more recovered asymptomatic people followed over longer periods of time are still needed to determine how long the antibody response to SARS-Cov-2 lasts, and what this means for re-infection and the design of effective vaccines, he said.
Antibodies are part of the immune response of what are known as B cells. Young said the other arm of the immune response, known as T-cells, have been shown to be present in recovered COVID-19 patients and should also be explored. These have been shown to last years in infections of other coronaviruses, he said.
Dr. James Gill, Honorary Clinical Lecturer at Warwick Medical School, U.K., and a practicing family doctor, said the study is a potential “warning shot to those who have been found to have antibodies to COVID yet have only had mild, if any symptoms.”
“Even those with a positive antibody test, especially those who cannot account for where they may have been exposed, should continue with to use caution, social distancing and appropriate mask use,” he said.
Mala Maini, Professor of Viral Immunology and Consultant Physician at UCL in the U.K., said not having detectable antibodies does not mean a person has no protective immunity. This is because they will likely have memory immune cells “that can rapidly kick into action to start up a new immune response if you re-encounter the virus. So you might well get a milder infection.”
Stephen Griffin, Associate Professor in the School of Medicine, University of Leeds, said: “Vaccines in development will either need to generate stronger and longer lasting protection compared to natural infection, or they may need to be given regularly.
“Whilst studies in primates show relatively weak responses that protect against severe disease but not infection itself or subsequent virus production, recent studies in pigs show that additional inoculations might improve outcomes.”
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