Assistant Professor Vivian Liu

In December, we put a call out for questions about COVID-19. We received a number of questions, and are publishing the first two responses this week. The responses come from Qian (Vivian) Liu, an Assistant Professor at the Institute of Parasitology and member of the McGill Centre for Viral Diseases. Her research focuses on understanding the virus-host interactions during the infection and transmission of emerging zoonotic viruses (such as SARS-CoV-2).

1) What is the data regarding the ability of COVID-19 to spread between vaccinated individuals? How easily does it spread and how does it compare to unvaccinated individuals?

Liu: “The variants of concern can partially escape the protection generated by vaccination or infection and spread between vaccinated individuals. In general, the variants of concern, e.g. omicron can spread faster than the parental strains of sar cov 2 wuhan-hu-1. The reason is that the variants of concern can emerge because they accumulate mutations that help them to better infect cells and escape the immune response at the same time. The spread of covid between vaccinated is slower than that of the unvaccinated individuals. The immune response in vaccinated people, although sometimes compromised towards variants of concern, still can provide some protection and kill some viruses during the transmission.”

2) There is immunity from the vaccine and natural immunity from having had the virus. What is the rationale for insisting people have two shots even if they’ve already had COVID?

Liu: “From the perspective of our immune system, the immune response generated by natural infection and vaccination is not of a great difference. In both cases, our immune system sees parts or whole of the pathogen (sars cov 2 virus) and makes antibodies and other immune responses against them. In this case, the vaccine can be seen as a mock virus to trick our body to generate an immune response against the real virus. So, regardless of the number of shots or natural infection that the person has already had, one more can always boost a stronger immune response against the virus. This has been supported by a few clinical studies published in nature etc. early 2021 when the gamma variant was prevalent.”