Building herd or mass immunity slows or stops the spread of an infectious disease within a population, usually through vaccination.

One out of five people in New York City and one in 10 in Wuhan have been infected by the coronavirus disease (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. If you look at the curves and draw a perpendicular from the peak, half the cases are before the peak and half are after. It’s a mirror image, and herd immunity is not achieved at the peak, but when the curve hits the X-axis after numbers decline,” said Dr T Jacob John, former professor of virology and emeritus professor at Christian Medical College, Vellore, Tamil Nadu.

Serological or blood tests can be done at the point of care for mass testing of people at risk, including health workers, to identify those who are already immune, so they can be deployed to care for infected patients to minimise the risk of Covid-19 outbreaks in hospitals.

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, which are antibodies that appear roughly after the third week. One Elisa-based IgG kit can test 90 plus samples in one go, so it’s an excellent tool for surveillance,” said Dr John.

Community surveillance of the general population can track if the pandemic has spread from urban to rural areas in India flowing mass migration from cities. “Multiple asynchronous epidemics are happening in India, so the peaks will vary, but we believe it will be in end July-early August,” said Dr John.

Antibody test kits arrived in India on April 16 after several delays, but the Indian Council of Medical Research (ICMR) asked states to suspend antibody testing on April 27 following its evaluation of kits of Guangzhou Wondfo Biotech and Zhuhai Livzon Diagnostics in field conditions showed results with wide variations in sensitivity. Apart from surveillance to detect asymptomatic cases, antibody tests help screen healthcare workers and others in essential services to determine if they have developed antibodies and can continue or return to work.

Since then, the Central Drug Standard Control Organisation of India has approved USFDA-approved testkits from pharma companies Roche Diagnostics and Abbott Laboratories, which are expected to arrive in India by the end of the month.

The Roche test, called Elecsys Ant-SARS-CoV-2, has 100% sensitivity and can identify Covid-19 antibodies in the blood sample of someone exposed to the disease in less than 20 minutes two weeks after the infection. Early data from an ongoing study funded by Abbott Laboratories showed its ID NOW rapid test, which is also used in the White House, accurately detects the virus 94.7% of the time, and correctly gave negative results 98.6% of the time, the company said in a statement on Thursday.

“Herd immunity will not happen till the infection peaks and falls, so until we have a vaccine, masks, hygiene, and social isolation are the only protection from infection,” said Dr John.

Sanchita Sharma

Courtesy Hindustantimes