Among all the things we don't know about COVID-19, one thing has been apparent from the start: it's far more dangerous to the old than the young. The reasons for this are less obvious, and a new study suggests one theory may actually have the truth backward. Rather than offering a low level of protection against the new disease, prior exposure to related viruses may actually undermine the immune system's capacity to defeat it.
SARS-1 was even more deadly, though less transmissible than SARS-CoV-2. However, most other members of the coronavirus family that infect humans cause nothing more serious than the common cold, indistinguishable by symptoms from the more common rhinoviruses.
Most of us have had many of these minor coronaviruses in our lives, and the older we are, the more we have had. Some non-scientists interpreted a study showing this past exposure had left many people with antibodies against COVID-19 as evidence of pre-existing immunity. However, Dr Amy Chung of the Peter Doherty Institute told IFLScience the main interest among virologists in these antibodies early on was whether they accounted for false positives in COVID testing.
Children get colds more often than adults, even though they may be less affected, thanks to more gregarious lifestyles. Chung thought this might be a contributing factor to differences in COVID-19 outcomes. She now thinks she had that part right, but the reason backward.
“Before commencing this project, we hypothesised the reason behind the differences in immune responses to COVID-19 were because children were likely to have more recent exposure to seasonal coronaviruses and therefore have some immunity to protect them against COVID-19,” Chung said in a statement. “But it’s the complete opposite. The elderly had more mature antibodies (known as IgA and IgG), which indicates repeated exposure to coronaviruses. Their immune system retains an immunological memory, and likely recognized COVID-19 as a less serious coronavirus, and were unable to mount a robust immune response."
Having cried wolf once too often and become used to coronaviruses not being serious, the immune system is taken by surprise when something that appears similar is actually deadly.
Chung and co-authors have published their conclusions in Nature Communications. She admitted to IFLScience that her samples of just 50 COVID-19 patients and the blood of 244 healthy people of various ages were not necessarily conclusive. “One of the benefits of living in Australia is that we have had so few cases,” Chung told IFLScience, “However, it has meant recruiting children is time consuming.” Nevertheless, some aspects of the work have been verified by larger international studies.
Moreover, Chung's conclusion is in keeping with an increasingly popular explanation for why influenza usually hits the elderly hardest. It's not just that the immune system is in decline, it is also that, when exposed to a novel form of 'flu it becomes “distracted” in Chung's words by the regions of the virus that look familiar.
There is no point trying to insulate yourself from all coronaviruses, Chung added. “The focus should be on vaccinating the elderly as quickly as possible.” Instead, she thinks her work's application, if verified, will be in improving vaccine design.
Chung has shown children's immune systems don't just block the virus, they clear it, and this is part of the capacity lost as we age. “Most vaccine studies just look at antibody response, not at whether it is actually clearing the virus away. This is something we are assessing with vaccinated individuals,” she said.