A security worker wearing protective clothing checks the temperature of a woman at a subway station entrance in Beijing as part of the effort to curb the Covid-19 epidemic. Photo: AFP

Theories about herd immunity and lifelong protection against Covid-19 after infection or vaccination have taken a hit by a recent study into a reinfection case.

The case, involving a patient in Hong Kong who recovered in April but caught the virus again after returning from Spain, has been officially confirmed by a University of Hong Kong study as the world’s first reinfection. 

The 33-year-old tested positive in Hong Kong in March and was discharged in mid-April after returning two negative results. 

Thinking he could not succumb to the virus again and wanting to take advantage of rock-bottom air travel and accommodation prices, he flew to Spain and traveled around the country this month. Spain was one of the states hit hardest in the previous wave of infections sweeping Europe. 

He had his saliva samples taken for testing at Hong Kong’s airport upon his return last week and was subsequently confirmed infected, despite displaying no symptoms. He was discharged last Friday after a speedy recovery.

Nurses and doctors treating Covid-19 patients applaud in front of a hospital in Barcelona, Spain, on April 5, 2020. Photo: NurPhoto

Genetic sequencing by HKU’s Department of Microbiology of samples from his first and second infections revealed two entirely different strains of the virus, with 24 different nucleotides.

Strain comparison against the coronavirus’ known gene pool suggested that the patient could have come down with the virus for the second time in Europe, not as a result of residual pathogens from his first infection. 

The HKU research team, supervised by former Assistant Director-General of the World Health Organization Professor Keiji Fukuda, announced earlier this week the key findings of this pioneering study. 

When the patient was hospitalized in a negative pressure ward for the second time after his European tour, initially epidemiologists could not find any trace of antibodies until five days after his admission.  

Experts at first were unsure if the man was a “persistent carrier” of the virus from his previous infection. 

HKU’s Chief of Infectious Disease Division Professor Ivan Hung, who took part in the research, pointed out that antibodies produced during a previous infection could only identify and target the same S protein of a virus to trigger an immune response but that process could be stymied if there was a mutation of the S protein.

Hung added that the team found at least four different amino acids on the S protein of the virus samples collected from the patient during his two infections.   

He said that in about six months since the first infection antibody levels of a recovered patient would drop to the extent that may not be able to fight off the same virus of a different strain.

“Many believe that recovered Covid-19 patients have immunity against re-infection because most developed a serum neutralizing antibody response. However, there is evidence that some patients have waning antibody levels after a few months,” the researcher said. 

“What we have learnt from this particular case is that immunity acquired through a previous infection or through vaccination like injecting an attenuated form of the virus may not endure past a year,” said the scholar, adding that for the best protection, people may have to receive two to three jabs annually once vaccines become widely available. 

Keiji Fukuda. Photo: naturemicrobiologycommunity.nature.com

Professor Fukuda, the former WHO official who is also dean of HKU’s Faculty of Medicine, also said the virus would be here to stay, just like other cold-related common coronaviruses and flu viruses. Individual and herd immunity may be too short-lived after natural infections and vaccination to stem the continued spread of Covid-19, he said. 

Fukuda said recovered patients who came down with the virus again may only have mild ailments or may be asymptomatic but they may still pass the pathogen to close contacts. 

“Anyone who have already survived Covid-19 should not have the false sense of safety and should abide by all the anti-virus rules, including social distancing and wearing a mask, and he should be properly vaccinated just like anyone else,” Fukuda said. 

“The findings have paint a gloomy picture that the fight against Covid is a protracted war and it may be a fallacy wishing to stamp it out at just one stroke. Hospitals, drugmakers and the entire healthcare sectors across countries will have to grapple with reinfections and the constant task of inoculation,” said Fukuda.

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