The infectiousness of SARS-COV-2
viral load, which leads to the transmission of infection in many others. But such families will be few, and 85% people with mild disease will not have secondary cases in their families.
A similar phenomenon is observed in epidemics caused by other new viruses, such as chikungunya, that saw 1.4 million cases and thousands of deaths in India in 2006. Even then, not all family members were affected and many people in the community did not fall ill even though the mosquitoes that transmit the disease were ubiquitous.
The explanation there also seems to be that many people have resistance to the new infection and individual-centric immunity or resistance plays a critical role in the transmission of the disease in a population even in the case of new infections.
What is the implication of the finding that a large proportion of the population is resistant to Covid-19? British neuroscientist Karl Friston from University College, London, has modelled data from many European countries and proposed a new concept called “Immunological Black Matter”. He suggests that a large proportion of the population is not exposed to the virus or are not susceptible to the infection. The practical implication of this is that with 20-30% infection rates or sero-prevalence in the community, the city or area will start seeing effects of herd immunity as about 50-60% of the population is not susceptible to the infection. It seems this is what is seen in cities such as Ahmedabad (central zone) where the sero-prevalence has reached 28%, and in Delhi where the sero-prevalence has reached 23% in July.
This does not mean that people should stop taking precautions to prevent transmission as social distancing and masks must be used until the epidemic completely disappears. But given the fast spread of this largely asymptomatic infection, other cities will follow the course of Ahmedabad and Delhi and will soon reach effective herd immunity.
Conducting more sero-surveillance in various cities will give a better understanding of the infection prevalence and how far we are from effective herd immunity. We need local epidemiological and public health studies to understand the disease dynamics. The “vocal for local” call by Prime Minister Narendra Modi should also be interpreted to mean generation and use of local data for disease modelling, and not just using global and imported models.