These are very early days in terms of understanding Omicron variants. What is known is that it carries a large number of mutations, particularly in the spike protein, and seems to be spreading rapidly in certain parts of the world. Very early indications from Africa suggest that it does not cause particularly severe disease (although the World Health Organization has urged caution given the limited data available). At this point, it is unclear whether it has any greater potential for avoidance of vaccines than other SARS-CoV-2 strains such as Delta. It is very common for the virus to become less effective (i.e. cause less severe disease) once it has become established in a population. The best example is myxomatosis, which killed 99% of rabbits when it first emerged in Australia, but is now less effective and causes a very low mortality rate. Some experts have predicted that COVID will also become less severe as it transmits an endemic level of disease – settling in a predictable pattern of infection at a particular location. It is possible that the Omicron version is the first step in this process. Why Some Variants Predominate Evolutionary biology suggests that variants are more likely to thrive if they multiply more rapidly in human populations than current strains.
A variant with less severe COVID symptoms expected to flourish?
This means two things: strains with a higher R number (the original breeding number, or the average number of people infected by an infectious individual) will replace those with a lower R number. Additionally, strains that make the host first infectious will replace strains that take longer to become infectious. Viral strain evolution needs to consider the particular population in which the variant appears. Disease development is expected to work differently in populations with low levels of vaccination than in those with higher levels of vaccination. Countries with a largely unvaccinated population, such as South Africa, where about 25% of the population has been vaccinated and where the Omicron variant was first detected, offer strains with higher R numbers a better chance of being effective. But in populations with higher vaccination rates, strains able to survive the vaccine will be more likely to dominate, even if they have lower R numbers than those without vaccination. Less severe symptoms can spread disease So, would you expect a variant with less severe COVID symptoms to flourish? It really depends on the interrelationship of traits and communicativeness. If symptoms are less severe, people are less likely to come forward for testing and therefore less likely to be isolated. Some people may not even realize that they have Kovid. Therefore, a variant with less virulence (meaning the body has less ability to cause severe symptoms) may be able to transmit to more people than strains with a higher virulence. On the other hand, as appears to be the case with Delta, some types can cause higher virus levels than others – meaning higher levels of virus in the bodies of infected people. The more viruses that are present, the more likely the person will be able to successfully transmit the disease.
Viral transmission a complex multistage process
Again, all things being equal (without knowing yet how specific mutations behave), higher levels of the virus are likely to cause more severe symptoms. It is not yet clearly understood why Omicron is apparently highly transmissible, at least in the African context, so at this stage we do not know whether it produces higher levels of virus than other strains. Viral transmission is a complex multistage process, so several things may be responsible for Omicron’s high transmission rate. Watch and wait What will happen next is not decided yet. Experts will seek more information on Omicron’s transmissibility, the level of virus it causes, and the extent to which it is able to evade existing vaccines or immune responses as a result of previous infections. Omicron may behave quite differently in a highly vaccinated population – as we are now seeing in Australia – than in a population with very low levels of vaccination as is the case in most of sub-Saharan Africa. Nevertheless, the emergence of this new version emphasizes that an effective worldwide vaccination effort is essential to overcome the COVID pandemic. Hamish McCallum, Director, Center for Planetary Health and Food Security, Griffith University South East