With Covid cases nearing a lakh, India to check prevalence of ‘asymptomatic’ carriers

The ICMR will conduct a 'serosurvey' across red, orange and green zones. The results of the study could see a re-designation of districts under the three zones.

Migrants wait in a queue to board trains to Assam and Meghalaya from Central Railway Station in Chennai during the nationwide Covid-19 lockdown (Representational image) | R Senthil Kumar | PTI

New Delhi: The Narendra Modi government has kicked off an exercise to check the prevalence of ‘asymptomatic’ cases of Covid-19 infections across India, ThePrint learnt.

All the three zones across India — red, orange and green — will be monitored under a new initiative of the Indian Council of Medical Research (ICMR) called “serosurvey” which will check the proportion of individuals who never had symptoms of the infection.

On Tuesday, the ICMR, the country’s apex health research body, announced that it will be conducting the serosurvey along with the Ministry of Health and Family Welfare, National Centre for Disease Control, state health departments and the World Health Organization (WHO).

With the cases of infections nearing one lakh, the government is concerned if the absence of symptoms among carriers is making the disease more contagious and difficult to control.

As of Thursday morning, there were more than 49,200 active cases with a total of over 75,000 infection cases.

No study so far

According to the WHO, 80 per cent of new Covid-19 cases are mild or asymptomatic. However, figures can be different across countries based on factors such as age and prevalence of high risk factors.

Raman.R. Gangakhedkar, chief epidemiologist at ICMR, had earlier said that “out of 100 people with infection, 80 do not have symptoms”.

However, sources in ICMR said the numbers were based on observations and till now, India hasn’t undertaken any large and formal study to ascertain the number of asymptomatic cases.

“These samples will be collected from the three zones across the country and all zones will have different surveillance strategies and benchmarks. We will study the proportion of asymptomatic cases in India to understand the disease’s prevalence,” Nivedita Gupta, a senior scientist at ICMR told ThePrint.

For instance, she explained, “Under survey, we drew samples from two different clusters. Sample A is from a non-hotspot where active cases are less in number. Sample B is from hotspot where active cases are more.”

“But, the antibody tests in sample A showed 15 per cent people having IGg antibodies and sample B showed just 5 per cent people having. It could mean the non-hotspot had (more) asymptomatic individuals than at hotspot,” she said.

“It means the chances of infection spread could be high in non-hotspot in the coming days,” she added.

IGg antibodies are developed in the human body after a person recovers from the infection.

Results may change designation of zones

In the later stages of the survey — depending on the final testing results and trends — the government may consider changing the designation of zones across the country.

Currently red zones are those with a higher number of active cases and take into account doubling rate, the extent of testing and surveillance. Orange zones see new cases occur after 14 days while green zones are areas that do not report a new case for 21 days.

Depending on the prevalence of asymptomatic cases, a green zone could be re-designated as an orange zone, and vice versa.

However, Gupta refused to comment on the zoning shuffle criteria, adding that “it would involve reading a lot of mathematical and scientific models”.

“Nothing is finalised as of now,” she said.

The government has maintained that there is no community transmission of the disease in India though there have been clusters with large outbreaks.

“The survey will check the community transmission to an extent, but its other objective is also to check the status of silent carriers in India,” Gupta said.

The Union health ministry has divided the more than 700 districts across the country into red, green and orange zones. There are 130 red districts, 284 that fall under the orange zone and 319 in the green zone.

Survey methodology

ICMR said the survey will be based on random samples collected from 69 districts of 21 states in India, which include 16 red zone districts, 31 orange zone districts and 22 green zone districts.

It will be a household level cross-sectional survey that covers 24,000 adults, distributed equally across four types of districts categorised on the basis of reported cases of Covid-19, the government announced.

The survey, according to the government, will involve collection of venous blood samples from 400 randomly selected individuals (one per household) from 10 clusters in each district. Sera from these individuals will be tested for presence of IgG antibodies using the ELISA test developed by ICMR-National Institute of Virology (NIV), Pune. The results of the survey will provide information about the spread of SARS-CoV-2 infection in different parts of the country.

Global status of asymptomatic cases

It is important to understand the prevalence of asymptomatic individuals as they are likely spreading the infection to vulnerable people unknowingly.

According to European Centre for Disease Prevention and Control, “no significant difference in viral load in asymptomatic and symptomatic patients has been reported, indicating the potential of virus transmission from asymptomatic patients”.

Different countries have shared different numbers of silent carriers across the globe ranging between 5 per cent to 80 per cent.

As per a Chinese study, a large majority of coronavirus infections do not result in symptoms. “A total of 130 of 166 new infections (78%) identified in the 24 hours to the afternoon of Wednesday 1 April were asymptomatic, said China’s National Health Commission,” according to the study published by The British Medical Journal.

However, another study conducted in Beijing found only 5 per cent of patients were asymptomatic out of a total 262 patients in a Covid ward.

In India, Delhi Chief Minister Arvind Kejriwal had shared state-level data which showed that nearly 75 per cent patients were asymptomatic or had mild symptoms.