Prior to late 2019, coronaviruses were best known as one of the main causes of the common cold. Coronaviruses typically only cause mild symptoms which last just a few days. The arrival of SARS-CoV-2, which causes COVID-19, certainly changed that, raising the profile of coronaviruses from seasonal annoyances to potentially deadly infections.
There are four coronaviruses known to cause cold symptoms – 229E, NL63, OC43, and HKU1 – the first are alpha coronaviruses and the second two are beta coronaviruses. These four coronaviruses are very common and all four are responsible for seasonal colds.
SARS-CoV-2 is not the only potentially deadly coronavirus. Two other coronaviruses cause serious infections that are potentially deadly: SARS-CoV and MERS-CoV. All three of these potentially deadly viruses are beta coronaviruses, and prior to the SARS-CoV-2 pandemic were rare. SARS-CoV and MERS-CoV outbreaks remained fairly localized, with the former in China and four other countries and the latter confined to the Middle East.
There is an immune response to coronaviruses which provides protection against reinfection with the same or even similar strains of a virus, so it is conceivable that previous exposure to cold-causing and SARS/MERS coronaviruses may offer a degree of protection against SARS-CoV-2 as a result of immune cross reactivity. Now some evidence has emerged that suggests that is the case.
A team of scientists in the UK detected antibody-driven immunity against the SARS-CoV-2 virus in some individuals who had not previously been infected with SARS-CoV-2 at the time of testing. The antibody response came from previous exposure to cold-causing coronaviruses.
The researchers found cross-reactive IgG antibodies in a small number of adult individuals – 16 out of 302 individuals tested (5.3%). These antibodies were believed to have been generated in response to infection with the common 229E, NL63, OC43, and HKU1 coronaviruses. Interestingly, in children aged 1-16 years, there was a much higher number of individuals who had SARS-CoV-2 S-reactive IgG antibodies – 21 out of 48 individuals tested.
The SARS-Cov-2 IgG reactive antibodies cross-reacted with the subunit S2 of the spike protein of SARS-CoV-2, which is used to gain access into cells. This part of the spike protein is thought to be similarly structured across the different types of coronaviruses, compared to the S1 subunit.
The researchers note that infection with SARS-CoV-2 tends to result in the generation of antibodies against both the S1 and S2 subunits, along with the generation of IgM and/or IgA antibodies.
Using cell cultures and sera from adults and children with the SARS-CoV-2 reactive IgG antibodies, the researchers found that the antibodies were able to neutralize both SARS-CoV-2 and SARS-CoV-2 S pseudotypes; however sera from uninfected individuals without the antibodies had no neutralizing activity.
The researchers suggest further research into targets on the S2 subunit could potentially lead to the development of a vaccine that would be effective against all types of coronaviruses.
The research also offers a potential reason as to why SARS-CoV-2 infection tends to be far less serious in younger people. Previous exposure to seasonal coronaviruses could be providing some protection against SARS-CoV-2, although it is unclear how long lasting that protection is.
You can read more about the study in the paper – Preexisting and de novo humoral immunity to SARS-CoV-2 in humans – which was recently published in the Science. DOI: 10.1126/science.abe1107