Mild And Asymptomatic COVID-19 Cases Have Some Long-Term Immunity To Virus

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New research has provided some good news when it comes to how long protective immunity against COVID-19 might last within people who only suffered mild or asymptomatic illness. 

In a new study, scientists report that 89 percent of healthcare workers still had neutralizing antibodies 16 to 18 weeks after infection, while most also had T cells capable of recognizing multiple different parts of the virus.

The findings of the new study are more or less in line with previous research on this issue. While it remains uncertain how long immunity lasts at this point – so far, most studies have only tested participants a few months after their infection – the findings provide some encouraging reassurance that the immune response to SARS-CoV-2 in asymptomatic infections may not be as short-lived as some feared. It’s also some positive news for the vaccination efforts which are currently being rolled out in parts of the world.

“Finally, here is the evidence of lasting antibody and T-cell immunity to SARS-CoV-2 that many had been waiting for,” Áine McKnight, study author and Professor of Viral Pathology at the Blizard Institute at Queen Mary University in London, said in a statement.

“A remarkable number of around 90 percent of individuals have a joint force of strong antibodies that prevent the virus from entering, coupled with T cell responses to various parts of the virus to interfere with its survival. This is an important find as mild or even no symptoms of Covid-19 are very common and representative of most infections in the community. Such abundant immune responses also give hope for the long-lasting efficacy of vaccines,” explained Dr Corinna Pade, study author and Postdoctoral Research Scientist at Queen Mary.

As reported in the journal Science Immunology last week, researchers studied the antibody and T cell responses of 136 healthcare workers from London hospitals, 76 of whom experienced an asymptomatic or mild SARS-CoV-2 infection. 

The immune response to COVID-19, as with any disease, is like a highly complex orchestra of multiple different types of immune cells and antibodies. In the simplest terms, it can be reduced down to two important types of white blood cell: B cells which make antibodies to identify and neutralize a pathogen, and T cells that are involved in cell-mediated immunity including directly destroying infected cells.

Interestingly, these new results also found peoples’ immune responses differed. The study discovered that many of the participants had “discordant” neutralizing antibody and T cell responses, meaning over half of the studied individuals had strong T cell immunity but weak antibody response, or vice versa. They also found that people with more "classic" symptoms of COVID-19, such as dry cough and fever, were more likely to have strong T cell responses. On the other hand, people with little to no symptoms tended to have a weaker T cell immunity than symptomatic infection, but similar neutralizing antibody responses.

Further studies have needed before scientists can reach any solid conclusions about the long-term immune response to COVID-19, but as it stands, many scientists are hopeful that people are capable of picking up a fairly robust and long-lasting immune response to SARS-CoV-2, whether that's achieved through natural infection or a vaccine. 

For more information about Covid-19, check out the IFLScience Covid-19 hub where you can follow the current state of the pandemic, the progress of vaccine development, and further insights into the disease.

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