Herd immunity is still far: Global data
One out of five people in New York City and one in 10 in Wuhan have been infected by Covid-19 and developed antibodies that protect them against reinfection, but the proportion of immune population is still too low even in outbreak epicentres for it to confer herd immunity that slows the spread of infection, according to data from several studies done around the world.
“Based on data from many countries that have done serosurveys, it is clear that only a small proportion of 5%-10% people have been exposed to this virus. It will be difficult to achieve herd immunity without a vaccine, the number of people who get ill or die may be unacceptably high,” said Dr Soumya Swaminathan, chief scientist, World Health Organization.
Building herd or mass immunity slows or stops the spread of an infectious disease within a population, usually through vaccination. It is an established public health strategy that works by vaccinating at least 70% of people within a community, who then don’t fall sick even when exposed to the virus. If a great majority of people are protected, there are very few unprotected people left to infect, which ends disease transmission and protects even those who are not protected.
“A number of seroprevalence studies (that look for antibodies to Covid-19 and assess the overall level of population exposure) have now been completed in both the US and elsewhere, and they suggest that outside of cities hit by the pandemic, overall infection rates and exposure remain relatively low,” tweeted @ScottGottliebMD, former commissioner, US Food and Drug Administration (USFDA), on Friday.
People who have experienced even mild coronavirus disease develop neutralising antibodies primed to seek and destroy the Sars-CoV-2 virus that leads to Covid-19 and protect them against reinfection.
“From what we’ve seen so far, the virus produces a strong immune response and the antibodies produced offer long-term protection, which is why the epidemic had declined wherever it has peaked,” said Dr T Jacob John, former professor of virology and emeritus professor at Christian Medical College, Vellore, Tamil Nadu.
With the National Institute of Virology in Pune developing an Elisa-based rapid test to detect IgG (Immunoglobulin G) antibodies for surveillance, it is possible to scale up antibody testing to determine the extent of the pandemic. “India so far is only testing for the virus, and not antibodies. We won’t know the extent of subclinical infection in the general population until we test everyone for IgG,” said Dr John.