Scientists in Asia are investigating whether countries with a frequent circulation of Sars-like viruses may have higher levels of generalised immunity among the population – a theory that, if proven, could help health authorities to manage the current Covid-19 pandemic.
The idea has been floated by medical experts including John Bell, a professor of medicine at the University of Oxford, who recently suggested that nations like Vietnam had avoided a flood of cases without a total lockdown because its citizens were not as immunologically “naïve” as first assumed.
This could mean that the regular existence of other Sars-like viruses had resulted in more natural resistance to such pathogens.
Until last weekend, Vietnam, a country of 95 million which neighbours China, had been a world leader in fighting the novel coronavirus, having eliminated local transmission of Covid-19 for 99 days.
The authorities are now battling an outbreak that spread unseen through the coastal resort of Da Nang. The first two Covid-19-related deaths were recorded on Friday, but the national case count still remained low, at 546, as of Saturday.
Professor Guy Thwaites, the director of the Oxford University Clinical Research Unit in Ho Chi Minh City, attributed Vietnam’s earlier success to its “extraordinary” response, particularly in terms of quarantine and the isolation of cases.
“In Vietnam they responded very quickly, they were able to track the first people coming into the country with it… [they were] all isolated, contacts all isolated,” he said. “They quarantined more than 200,000 people over a period of four months, even with relatively small numbers of cases.”
The idea of generalised immunity was “an entirely reasonable hypothesis,” but as yet there was no evidence that it was behind Vietnam’s ability to fight the virus, Prof Thwaites said.
“It’s a potential idea and it’s something that we can work on. We’ve done work on coronaviruses before. We have the ability to test that hypothesis quite easily and we will do.”
Scientists in Singapore which, like Vietnam, was badly hit by Sudden Acute Respiratory Syndrome (SARS) in 2003, are already actively exploring the concept.
“The possibility that in some rural areas there is a higher level of pre-existing immunity is something that we are actively studying,” Professor Antonio Bertoletti, a specialist in infectious diseases at the Duke-NUS Medical School, told The Telegraph.
Prof Bertoletti led a research team that recently uncovered the presence of virus-specific T cell immunity in people who recovered from COVID-19 and SARS, as well as some healthy study subjects who had never been infected by either virus.
T cells, along with antibodies, are an integral part of the human immune response against viral infections due to their ability to directly target and kill infected cells.
The findings suggest infection and exposure to coronaviruses induces long-lasting memory T cells, which could help in the management of the current pandemic and in vaccine development against COVID-19.
The revelation that T cells were found in all subjects who recovered from SARS 17 years ago, and in over 50% of those who had not had SARS or the current Covid-19, suggested a level of pre-existing immunity in the general population to SARS-CoV-2, the coronavirus strain that causes the Covid-19 disease.
Prof Bertoletti stressed there was a difference between the presence of immunity and protection from the virus, adding that the extent to which T cell responses were "protective" was still unknown.
“I am more optimistic and I do think that such level of T cell immunity will be able to confer some level of protection, but you need to remember that T cells do not act to prevent infection, they act to contain the spread of the virus within the host,” he said.
“It is possible that such a level of pre-existing immunity can be one of the causes that allow the majority of infected subjects to control the virus without severe disease.”
Ongoing research shows that asymptomatic patients also have a good level of T cell immunity, he said, adding that an analysis of the immunity levels in the population could reveal who was more at risk of infection or disease.
Dr Todd Pollack, country director at Harvard’s Partnership for Health Advancement in the Vietnamese capital, Hanoi, said immunity stemming from past coronavirus exposure theoretically might explain why people experienced severe or no symptoms and why some countries in Asia had less infections.
However, he warned against explaining away Vietnam’s successful pandemic prevention model.
“There seems to be a tendency to say there must be some explanation for why they are successful over there in Asia. In fact, I think probably the biggest explanation is the response that the government and health agencies took in these countries,” he said.
“Vietnam did most of the right things…its communication strategy to the population, mask-wearing, border control, contact tracing, quarantine. Those are the key, important parts. But most importantly they did them early and rapid.”