Silent COVID-19 reinfections have researchers worried
Jill Margo

Just as your first infection with the COVID-19 virus can go unnoticed, so can a reinfection, according to a case study of hospital workers in north India.

In a letter to the journal Clinical Infectious Diseases, published by Oxford University Press, the case study reports on two young staff members at a tertiary hospital who tested positive and became reinfected several months later.

Infections and reinfections in the two health workers were detected, on both occasions, through routine surveillance. They were feeling fine.  AP

While other cases of reinfection have been reported, all had symptoms in one or both of the episodes.

But these two health workers were unusual because they had no symptoms either time and both were infected with a genetically different virus the second time.

Reinfection raises serious questions about how long an immune response to an infection lasts and whether the world can rely on the immune system to end the pandemic.

The infections in the two asymptomatic health workers were detected, on both occasions, through routine surveillance.

The workers, a 25-year-old-man and a 28-year-old woman, worked in a COVID-19 ward and tested positive for SARS-CoV-2 in May.

Both were admitted to hospital as per institutional policy. They subsequently tested negative and resumed duties in the hospital.

About 3½ months later, routine swab testing found they were positive again.

Infected by new strain

While both had no symptoms, this time they both had higher levels of virus than the first time.

Their initial swabs had been archived and they agreed for them to be analysed for a comparison with their new infections.

A genomic analysis at the Institute of Genomics and Integrative Biology in New Delhi showed the virus that infected them the second time was genetically different from the first.

This means they were infected anew rather than harbouring leftover virus.

The authors say their cases suggest asymptomatic reinfection may be potentially under-reported and underlines the need for continuous surveillance of reinfections in healthcare systems.

New focus of attention

They also say the genetic difference in the virus the second time rules out persistent viral shedding or reactivation in these two cases.

Reinfection has become a focus of attention. In late August, when news broke about a man from Hong Kong being reinfected months after recovering from COVID-19, some experts were encouraged.

His second infection, detected at an airport, caused no symptoms. This suggested his immune system might have remembered the previous attack and fended off the second one before it could do harm.

Cases of reinfection in other countries quickly came forward, including one from Nevada, the first case of reinfection in the US.

The symptoms were more severe than the first time and had experts worried. Had the immune system malfunctioned and allowed things to get worse?

There are many unknowns about reinfection. Does the severity vary within the same person because of the size of the virus dose, the virus variant or because the individual’s health is different?

While the Hong Kong case may demonstrate immune memory, the Nevada case did not, raising the possibility the immune system can potentially cause damage.

Experts say reinfection does not mean a vaccine can’t be effective, but that boosters may be needed to maintain protection. Although inconvenient and costly, these would place long-term immunity against the virus within reach.

One worry, however, is that vaccinated people could be turned into asymptomatic carriers, putting others at risk.

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Jill Margo is the health editor. She writes about medicine and health from the Sydney office. Jill has won multiple prizes, including two Walkley Awards and is an adjunct associate professor of the University of NSW, Sydney. Connect with Jill on Twitter. Email Jill at

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