Relying entirely on thermal scanners to identify every novel coronavirus infected passenger has turned out to be a failed exercise. From the first student from Kerala who arrived from Wuhan in end January, every imported case is a testimony to the limitation of thermal screening.

On January 17, when the number of novel coronavirus (SARS-CoV-2) cases reported in China were just 41 with two deaths, India began screening passengers for a fever using thermal scanners at three airports — Mumbai, Delhi and Kolkata. Four days later it was extended to four more airports and subsequently to 30 airports.

In addition to the visa restrictions already in place, on March 14, India suspended all existing visas (except diplomatic, employment and project visas) till April 15. By then, nearly 12,30,000 passengers arriving in over 11,400 flights were screened for a fever using thermal scanners.

Limited success

Thermal screening did help identify over 3,000 passengers and nearly 600 contacts with a fever, who were then referred to designated hospitals. Another over 42,000 passengers had been brought under community surveillance, of which over 2,500 were symptomatic and 522 hospitalised.

Despite its success in identifying a few thousand infected passengers at the point of entry, relying entirely on thermal scanners to identify every infected passenger has turned out to be a failed exercise. Worse, thermal screening gave a false sense of security of having secured the country from the virus. Unfortunately, India failed to realise the limitations of thermal screening despite early warnings.

Every imported case is proof

The first student from Kerala who arrived from Wuhan in end January developed symptoms and tested positive for the virus a couple of days after landing in India. The story repeats itself in every single imported case. According to Covid-19 India tracker, a volunteer driven third party website, 193 of the first 300 people who tested positive for the virus in India had a travel history. The same applies to the foreign nationals who had participated in the religious congregation held in mid-March by Tablighi Jamaat in Nizammudin, Delhi and now tested positive for coronavirus.

The infrared fever screening system to quickly check for a fever made its debut in Singapore in 2003 after severe acute respiratory syndrome (SARS) outbreak. If proved futile during the SARS, only a small fraction of the infected were captured by thermal scanners during the 2009 H1N1 influenza pandemic in China and Japan.

Why thermal screening fails

Since the core body temperature is the temperature of blood in the heart and the brain and measuring the skin temperature can be misleading. Temperature of the temples, neck and a small patch of skin between the eyes and nose can come close to the core body temperature but is not always accurate. Add to it the role of fever suppressing drugs which can completely temporarily mask the symptom.

The incubation period is 7-14 days for the novel coronavirus. Since no symptoms show up during the incubation period, relying on thermal screening alone can be problematic.

Studies and experiences from other countries now point out that novel coronavirus can make thermal screening even more meaningless. The reason: a large percentage of infected people never show any symptoms or show only mild symptoms. Nearly 5% (nearly 18,000) of the population in Iceland have been tested for the virus and only 1% tested positive. Of these nearly 50% were asymptomatic or show only mild symptoms. Nearly half a dozen studies have shown that people without symptoms can spread the infection in the community. It Italy, the virus was silently spreading in the community for 50 days before the first case was reported on February 20.

China is now reporting more number of imported cases — 841 as on April 2 — and very few cases of local transmission despite every incoming passenger at all airports being checked for fever.

What is the alternative

Sealing the border to prevent any incoming passengers will be the most effective way to keep pathogenic microbes at bay. But that is an extreme measure and cannot be undertaken at an early stage of an outbreak. The only solution then is to quarantine every single passenger for the entire duration of incubation or institute measures to track people who self-quarantine at homes. Relying solely on thermal scanners to catch every infected case can be disastrous. India cannot repeat this mistake again when another pathogenic virus strikes.