Covid-19 triggers antibodies from previous coronavirus infections, says study

People with Covid-19 may rely on antibodies created during infections from earlier coronaviruses to help fight the disease, says a new study that may partially explain the difference in symptom severity between old and young patients.

The study, published in the journal Cell Reports Medicine, noted that humans have navigated at least six other types of coronaviruses before SARS-CoV-2 -- the virus that causes Covid-19.

"Our results suggest that the Covid-19 virus may awaken an antibody response that existed in humans prior to our current pandemic, meaning that we might already have some degree of pre-existing immunity to this virus," said John Altin, a co-author of the study from the Translational Genomics Research Institute (TGen) in the US.

In the research, the scientists used a novel tool called PepSeq to map the body's antibody responses to all human-infecting coronaviruses.

"The data generated using PepSeq allowed for broad characterization of the antibody response in individuals recently infected with SARS-CoV-2 compared with those of individuals exposed only to previous coronaviruses that now are widespread in human populations," said Jason Ladner, the study's lead author from Northern Arizona University in the US.

The researchers examined the antibody responses from two other potentially deadly coronaviruses -- MERS and the 2002-03 SARS pandemic virus.

They also characterised the antibody responses of four older coronaviruses -- alphacoronaviruses 229E and NL63 as well as betacoronaviruses OC43 and HKU1.

According to the scientists, these are common viruses that are endemic throughout human populations, but usually are not deadly and cause mild upper respiratory infections similar to those of the common cold.

By comparing how the antibodies react against these different coronaviruses, the researchers demonstrated that SARS-CoV-2 could trigger immune system antibodies originally generated in response to these past coronavirus infections.

They said the cross-reactivity with these antibodies occurred at two sites in the SARS-CoV-2 Spike (S) protein which enables the virus to enter and infect human cells.

"Our findings highlight sites at which the SARS-CoV-2 response appears to be shaped by previous coronavirus exposures, and which have potential to raise broadly-neutralizing antibodies," Altin said.

"We further demonstrate that these cross-reactive antibodies preferentially bind to endemic coronavirus peptides, suggesting that the response to SARS-CoV-2 at these regions may be constrained by previous coronavirus exposure," he said, adding that further research is needed to understand the implications.

The scientists believe the findings also explain the widely varying reactions Covid-19 patients have to the disease from mild to no symptoms, to severe infections requiring hospitalisation, and often leading to death.

They said the differences in the pre-existing antibody response identified by this study may possibly explain some differences in how severely Covid-19 disease affects old versus young people.

"Our findings raise the possibility that the nature of an individual's antibody response to prior endemic coronavirus infection may impact the course of Covid-19 disease," Ladner said.