A new study conducted by Massachusetts General Hospital showed that patients who develop a severe form of Covid-19 continue to have antibodies months after infection.
Survivors of severe forms of Covid-19 develop immune responses to SARS-Cov-2, which provides lasting protection against possible reinfection.
This is the conclusion reached by researchers at Massachusetts General Hospital (MGH) in a study published in Science Immunology. In their opinion, the measurement of these antibodies could prove to be an accurate tool to monitor the spread of the virus in the population.
Immunity for at least four months
In response to SARS-Cov-2 infection, the virus responsible for Covid-19, the immune system produces antibodies. But, explains Richelle Charles, lead author of the study, “there is a huge gap in knowledge about the duration of these antibody reactions.”
To find out how long the immune response against the new coronavirus lasted, Richelle Charles and her colleagues analyzed blood samples from 343 patients with Covid-19, most of whom had developed a severe form. The blood samples were taken up to four months after the onset of symptoms. The blood plasma was isolated and exposed to the virus. The team then studied how different types of antibodies in the plasma were linked to the virus. The results were compared with blood samples taken from over 1,500 people before the pandemic.
The researchers found that the measurement of an antibody called immunoglobulin G (IgG) was very accurate in identifying infected patients who had been symptomatic for at least 14 days. Since the standard PCR test for SARS-Cov-2 loses its sensitivity over time, researchers believe it is important to combine it with a serological test in patients who have been symptomatic for at least eight days. This will help identify some positive cases that may have been overlooked.
Tracking antibodies to monitor their spread
In addition, researchers found that IgG levels in these patients remained high for four months and were associated with the presence of protective neutralizing antibodies, which also showed a slight decrease in activity over time. “This means that people are most likely protected during this time,” said Professor Charles. “We have shown that the main antibody responses to Covid-19 do persist”.
While immunoglobulin G levels remained high in the months following SARS-Cov-2 infection, immunoglobulin A (IgA) and immunoglobulin M (IgM) levels dropped much faster falling to low levels on average within two and a half months or less. “We can now say that a patient who has both IgA and IgM reactions is likely to have been infected with the virus in the last two months.
For Dr. Jason Harris, MGH’s pediatric infectious disease specialist and co-author of the study, knowing the duration of the IgA and IgM immune response will help researchers obtain more accurate data on the spread of SARS-Cov2.
“There are many infections in the population that we cannot detect through PCR tests during acute infection, and this is especially true in areas where access to testing is limited,” he said. It is important to know how long antibody reactions last before antibody tests can be used to monitor the spread of Covid-19 and identify “hot spots” of the disease.