Reinfections of the new coronavirus in four healthcare workers in Mumbai with strikingly greater clinical severity during their second episodes have surprised sections of the medical community as they defy expectations of milder second-time coronavirus infections.

Medical researchers in Mumbai and New Delhi used genome sequence studies on virus samples from each of the four healthcare workers to establish they were true reinfections and not just leftover viruses from their previous infections.

The reinfections, the researchers say, carry a take-home-message for the public, relevant now to over 5.4 million people across the country who have recovered from Covid-19 — continue to take precautions.

All four healthcare workers, who are engaged in treating coronavirus disease patients in Mumbai hospitals, had either no symptoms or mild symptoms during their first episodes in May and June.

However, during their second episodes in July, they developed upper respiratory symptoms, muscle pains, or fever and all were hospitalised for observation and treatment. None of the four developed lung inflammation or disease that doctors have attributed to their relatively young ages.

Although, Covid-19 reinfections are rare — only around 10 confirmed through genome studies worldwide among over 27 million people who have recovered from the infection — the Mumbai reinfections are considered significant because they establish the potential for severe reinfections.

“Coronavirus reinfections are typically milder — that’s what we’d expect to see,” said Anurag Agrawal, a doctor-turned-researcher and director of the New Delhi-based Institute of Genomics and Integrative Biology (IGIB), a unit of the Council of Scientific and Industrial Research.

Parents worldwide are familiar with this phenomenon of milder subsequent coronavirus infections. Young children sent into daycare or playschools often show frequent bouts of respiratory infections that wane in both intensity and number over three to five years.

Agrawal and his colleagues have pointed out in a research report that while reinfections appear to be rare, it is unclear how long post-Covid-19 immunity will last.

“If those who recover from mild Covid-19 have short-term immunity, reinfections may become more common in the future,” they said in their paper not peer-reviewed yet but posted on a preprint archive server.

Doctors have so far confirmed Covid-19 reinfections through genome studies in Hong Kong and Nevada (US).

Researchers at the IGIB had last month also established reinfections in two healthcare workers in a government hospital in Noida.

The mechanisms that underlie reinfections are still unclear. Some scientists have speculated aberrant immune responses in some individuals might make them vulnerable to reinfections.

“The reinfections underline the importance of continued precautions such as masks even by people who have recovered from the infection,” Agrawal said.

“We don’t know enough about this virus yet.”

The research team included doctors at the Kasturba Hospital and the PD Hinduja Hospital in Mumbai and researchers at the IGIB and the International Centre for Genetic Engineering and Biotechnology, New Delhi.