Does infection with several of the four other common coronaviruses that circulate each winter protect against Covid-19? This question has been investigated in several studies of potentially frequent cross-immunity that already exists in the population.
Several studies published in recent days investigated the possibility that part of the population is already protected from the SARS CoV-2 coronavirus responsible for Covid-19 by what is known as cross-immunity. In other words, being infected with other viruses would unintentionally provide protection against Covid-19. When someone is cured of an infection, in most cases the immune system has developed antibodies to effectively fight the pathogen responsible for the infection. Cross-immunity refers to the fact that these antibodies are also effective against other viruses.
This was observed by a team of Swiss and American researchers using samples from a patient who was cured during the 2003 SARS epidemic. At that time, 8000 people (almost 800 deaths) were infected with the coronavirus SARS-CoV-1, which is 80% similar to the coronavirus Covid-19. In the journal Nature, a team led by Davide Corti identified a specific antibody, S309, which neutralized both SARS-CoV-1 and SARS-CoV-2. In 2003, S309 was extracted from the B cells of the so-called patient. These cells are produced during a past infection and are responsible for the production of specific antibodies: this is the immune memory. However, tests have so far only been carried out in vitro. Even more importantly, too few people were affected by the SARS epidemic for this cross-immunity to have an impact on the current pandemic.
60% of the population may already be protected
While SARS-CoV-1 disappeared for good in 2004, four other human coronaviruses (HCoV) circulate seasonally and are responsible for 15-20% of colds each year. Two have been known since the 1960s, 229E and OC43, while the other two, NL63 and HKU1, were discovered shortly after the outbreak in 2003. However, previous work has already shown that some of them are interdependent. For example, antibodies produced to fight OC43 during a cold can also have a neutralizing effect against HKU1 or even SARS-CoV-1. According to a study published in Cell on 14 May 2020, this crossover immunity can protect as many as 40-60% of the population from SARS-CoV-2!
In short, a person that had a cold that was caused by either OC43 or HKU1, may have some protection against SARS-CoV-2. But the figure of 40-60% seems very optimistic. This is because we also know that the protection provided against common coronaviruses usually lasts no longer than two years. How long exactly? The answer to this question is difficult because it depends on the immune profile of each individual. But these are viruses that you encounter several times in your life.
The scientists behind this glimmer of hope compared the immune response of patients cured of Covid-19 with the response of 20 non-affected people using samples taken between 2015 and 2018. In the latter group, all had antibodies against OC43 and NL63. Another team in Berlin similarly analyzed the immune response of 68 people who had never been exposed to Sars-CoV-2. Their first publication on Medrxiv should be treated with caution. But they found a 34% rate of the cross-immunity.
The uncertain impact of cross-immunity
For a growing number of researchers and doctors, this cross-immunity would be a significant advantage in preventing a second wave in the coming months.
It is still too early to know how cross-immunity with other coronaviruses will affect the course of the pandemic. While this cross-immunity has implications, it may not be as expected… Marc Lipsitch’s study reported in the journal Science last April: “Low cross-immunity with other betacoronaviruses can lead to the disappearance of SARS-CoV-2, followed by resurgence after a few years”. This is because the protective immune memory against these coronaviruses usually lasts no longer than two years. Thus, “moderate cross-immunity (30%) in combination with OC43 and HKU1 could effectively prevent the transmission of CoV-2-SARS for a maximum period of three years, before a resurgence in 2024,” researchers have already warned. Cross-immunity or not, if SARS-CoV-2 does not completely disappear like its close cousin SARS-CoV-1 in 2004, caution will still be needed for a long time.