India will soon start testing blood samples of people who do not have symptoms of the coronavirus disease in an attempt to gauge the scale of the outbreak.

The Indian Council of Medical Research, which is overseeing the country’s testing regime, is procuring 10 lakh kits for these serological tests. “We will start [testing] as soon as we get the kits,” said Nivedita Gupta, a senior scientist at the ICMR.

These testing kits will not be used by the ICMR for diagnostic purposes. To identify active Covid-19 patients, India employs the RT-PCR or reverse transcriptase polymerase chain reaction test on nasal or throat swab samples of suspected patients. The RT-PCR tests look for the presence of the virus in the DNA.

In comparison, serological tests identify disease-fighting antibodies in the blood, and are useful to map the spread of the virus among those who may not have shown any symptoms at all.

Rajni Kant Srivastava, director of the ICMR’s Regional Medical Research Centre, said the serological tests would be done as part of a study “to check the prevalence of Covid-19 among asymptomatic people”.

Nivedita Gupta explained: “Right now we don’t know what is the spectrum of the disease – how many people would be there in our community who have been infected but never had symptoms. This is the way to find that out.”

Looking for antibodies

Why is the ICMR using serological tests and not the regular RT-PCR test for the study?

“When you want to know how many people [in a community] were actually infected, the antibody test is much more useful as the window of opportunity is much larger,” said Gupta. This is because antibodies stay in a person’s body for a much longer time than the virus, she explained.

The incubation period of the novel coronavirus – the time it takes to show up in the body – ranges between 2-14 days. But the virus shedding period when an infected person has a heavy load of the virus in their body and can transmit it to others lasts a maximum of five days. “After that period, it is best to detect an antibody,” said Gupta.

How long do the antibodies stay? There are two types of antibodies, said Gupta. “By permutation and combination, say one year,” she said.

The serological tests would help understand “the burden of the disease”, the scientist explained. “You identify a community within a certain catchment area and that would give you a sense of how many were infected.”

Previous models

In 2017, the National Institute of Epidemiology did a similar survey to gauge the extent of dengue in India. The survey was done in 240 clusters – 118 of them rural and the rest urban – across 60 districts in 15 states.

Two villages formed a rural cluster while two municipality wards formed an urban cluster. The 240 clusters were randomly selected using an Android programme.

The survey findings indicated that 49% of India’s population had been previously infected by the dengue virus.

‘Very important tool in public health’

Gagandeep Kang, a clinical scientist who heads the Translational Health Science and Technology Institute, an autonomous government institute, said a similar survey for the novel coronavirus would help India make informed policy decisions.

Serological surveys, Kang said, were a “very important tool in public health”. She explained: “It tells you how much disease there is, it shows you patterns and you can use it put into models.” Mathematical modelling forecasts the future graph of the disease.

As this article in the Science Magazine explains, data from these tests “could inform practical issues such as whether and how to reopen schools”:

“Relatively few cases have been diagnosed among children, but it isn’t clear whether that’s because they don’t get infected or because their infections are generally so mild that they go unnoticed. Testing children for SARS-CoV-2 antibodies should resolve that.”

The test, the article adds, “may also help researchers understand how long immunity to the virus lasts, a key issue for any future vaccine”.

Should the ICMR have started the serological tests earlier?

Kang, who has been a critic of the ICMR’s restrictive testing regime, said she had no complaints this time. Reliable serological kits have only recently become available in the global market, she said. In fact, large-scale data on their efficacy was still not in the public domain.

Rapid antibody kits

On March 27, the ICMR released guidance for the use of rapid antibody testing kits in India, which said “positive test indicates exposure to SARS-CoV-2” but “negative test does not rule out COVID-19 infection”. This is what makes the serological tests less reliable compared to the RT-PCR test for the purpose of diagnostics, that is, confirming whether a person is infected. The guidance says the kits are “not recommended for diagnosis of COVID-19 infection”.

The ICMR statement also listed 12 antibody testing kits which had approvals: 11 had the Conformitè Europëenne, the European Union’s quality benchmark also known as European CE. Two of the kits – one with an existing CE marker – had been evaluated and approved by Pune’s National Institute of Virology.