An article in science Much interest has recently been generated by providing a possible explanation for why COVID-19 can be fatal to some but goes virtually unnoticed to others.
Scientists at California's La Jolla Institute of Immunology showed that cold coronavirus infection can elicit an immune response similar to key elements of that produced by SARS-CoV-2 – the virus that causes COVID-19. This increases the possibility that previous infection with one of the milder coronaviruses could make COVID-19 less severe. But how likely is that? And how does this relate to what we already know about coronaviruses?
A few weeks ago a other article sat at the center of the SARS-CoV-2 immunity debate. This showed that the antibody response to SARS-CoV-2 can decrease over time.
The results raised concerns that SARS-CoV-2 could infect a person many times and that a vaccine may not produce permanent protection. However, the article only focused on one arm of the immune response, the B cells, which produce antibodies that help clear an infection.
T cells are also the key to the immune response against viruses. They play a variety of roles, including helping B cells mature into disease-fighting machines. The article by Jose Mateus and colleagues at the La Jolla Institute of Immunology is important as it shows that people are keeping T cells away from the milder coronaviruses long enough to potentially interact with a new challenge posed by SARS-CoV-2, and that these T cells may even recognize SARS-CoV-2 and help clear the infection.
The case for cross immunity
The evidence of waning immunity and cross immunity came as no surprise to epidemiologists. A study from 1990 showed that soldiers infected with any of the milder coronaviruses were immune for no more than a year. Also the boom-bust cycle that the milder coronaviruses go through from year to year can be explained through a mixture of dwindling immunity and cross-immunity.
The milder coronaviruses can produce antibodies similar to those produced by the coronaviruses cause Sars and Mers. These antibodies are so similar that they are almost tricked A care facility in British Columbia believed they had a Sars outbreak after the Sars epidemic was declared over. In fact, the outbreak was caused by OC43, one of the coronaviruses that causes the common cold.
Infections that produce structurally similar antibodies, however, do not necessarily offer medically meaningful cross protection.
We're still not sure
There is little evidence of cross protection between all but the most closely related coronaviruses.
It's hard to say whether the milder coronaviruses protect against SARS-CoV-2, also because we monitored them so little. Ideally, we could use historical data to determine which communities have had major outbreaks of each milder strain of coronavirus in recent years, and then determine whether there is a link to less severe COVID-19 cases.
Challenge studies in which a person is intentionally infected with a milder strain of coronavirus and then exposed to SARS-CoV-2 could also answer the question, but are dangerous and ethically questionable. At the moment we can only say that the possibility that the common coronaviruses protect against SARS-CoV-2 remains exactly that – a possibility. Indeed, Mateus and colleagues describe this theory as "highly speculative".