Since COVID-19-causing virus, the novel coronavirus is new and not much is known about it. Scientists are trying to figure out if getting the disease once can protect you from reinfection – something that normally happens in most infectious diseases.
Widespread antibody tests are being done in some areas to look for the presence of COVID-19 specific antibodies in the population to check for the possibility of a secondary infection and to trace the spread of the disease. About 80 percent of the cases of COVID-19 are mild or asymptomatic so it is likely that a lot of people may have gotten the disease and have recovered without even realizing.
A group of researchers at the Karolinska Institutet and Karolinska University Hospital, Sweden have found that immunity to COVID-19 may be more widespread in the general population than an antibody test can tell. And hence there might be a need to change our tactics for tracing immunity responses against the disease.
The study is still in the pre-print phase and is not yes peer-reviewed
The evidence so far
Normally, when a person gets a disease, their immune system makes antibodies to fight the pathogen. After recovering, some of these antibodies stay in our body to recognise and quickly eliminate the same pathogen, in case we get exposed to it a second time.
Antibodies are specific to a pathogen. So an antibody against one pathogen may not protect you against another. An antibody test helps look for the presence of disease-specific antibody molecules in a patient’s body.
However, one of the major questions that we don’t have a certain answer to is whether a person becomes immune to the COVID-19 causing virus once they get exposed to and recover from it.
Though some animal studies show that the immune system makes antibodies against the SARS-CoV2 virus which can protect from reinfection, there have been previous cases of a second infection in recovered patients. Also, it has been found that those who get mild disease do not even have detectable levels of antibodies.
A study with common cold causing coronaviruses even showed that the antibodies in patients decline up to 50 percent within 6 months.
The new study
For their research, the scientists at Karolinska University studied over 200 patients in Sweden with either mild/asymptomatic or severe infection.
They found that all convalescent patients (those who were recovering from the disease) had robust T-cell mediated immunity even in the absence of antibodies. Exposed family members and about 30 percent of healthy individuals who donated blood in May 2020 were found to have T-cell immunity against the infection as well. Though, those with severe disease had a more robust T-cell response than those with mild or asymptomatic disease.
Interestingly, T-cell responses have been reported after immunization in some of the successful vaccines like that of yellow fever.
Antibodies are not the only type of cells that our immune system makes to fight an infection. T-cells are another type of cells that help fight pathogens, especially intracellular (those that live inside body cells) ones like viruses.
Virus-specific T-cells have been noted in patients years after infection with SARS virus (SARS-CoV-1).
The study mentioned that it is not known yet if these T-cells can protect the person from reinfection with SARS-CoV-2 in the absence of antibodies. Nonetheless, previous studies with MERS and SARS virus (two related coronaviruses) have shown that T-cells are indeed effective against a secondary infection even when antibodies are not present.
For more information, read our article on Immunity to COVID-19.
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