In Depth

Why Covid herd immunity could be ‘twice as high as first thought’

Scientists say some people may be resistant even if they haven’t had the new coronavirus

Two new scientific studies have offered some much-needed hope in the battle against Covid-19, by suggesting that immunity to the disease may be much higher than previously thought.

Although coronavirus antibody testing has found that levels are low even in the cities worst hit by the pandemic, other forms of immune response seem to be more widespread.

Indeed, “public immunity could be as much as twice that found in antibody tests, meaning infection hotspots such as London could be further along the path to herd immunity than thought”, says The Telegraph.

A natural ceiling?

Scientists have been puzzled by the tendency of the new coronavirus to spread rapidly but then slow down long before they would expect to see herd immunity.

“On the Diamond Princess cruise ship, for example, where the virus is likely to have spread relatively freely through the air-conditioning system linking cabins, only 20% of passengers and crew were infected,” say Sweden-based epidemiology professors Paul W. Franks of Lund University and Joacim Rocklov of Umea University, in an article on The Conversation.

The 20% ceiling on infections has also been seen in outbreaks on military ships and in London, Stockholm and New York, they note.

“This has led to speculation about whether a population can achieve some sort of immunity to the virus with as little as 20% infected” - much lower than the “widely accepted herd immunity threshold” of 60% to 70% suggested by earlier mathematical models.

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How T-cells could explain the discrepancy

Recent studies have focused on the role of T-cells, a type of white blood cell that forms part of the human immune system.

T-cells “thwart infections in two different ways”, says Science magazine: by spurring other cells to produce antibodies and by targeting and destroying infected cells. The severity of disease can depend on the strength of these T-cell responses.

And unlike antibodies, which are specific to a particular virus or bacteria, T-cells activated by one infection can protect against an array of similar threats.

As such, people with activated T-cells that are effective against the Covid-19 coronavirus could help to close the gap between the 20% of people with antibodies and the 60% or more needed for herd immunity.

‘A third of us may be immune’

A new study conducted by the Karolinska Institutet in Sweden looked at blood samples from 200 adults, including people with no symptoms of Covid-19.

“In total, 30% of the healthy blood donors were found to have developed ‘T-cell immunity’ – twice the number of cases in which antibodies were detected,” The Telegraph reports.

Whether active T-cells - or indeed antibodies - offer complete protection against the coronavirus is unclear, but scientists say that both responses are likely to help the body fight the virus.

“What this means is that we are probably underestimating the number of people that have some type of immunity,” said study co-author Marcus Buggert, an assistant professor at the institute’s Centre for Infectious Medicine.

‘Four in five have some protection’

Another study involving T-cells by researchers at Germany’s University Hospital Tubingen found even more widespread immunity among a total of 365 volunteers.

When their blood was exposed to the new coronavirus, the 180 participants who had previously had Covid-19 showed the strongest T-cell immune response, as had been expected.

“But surprisingly, there was also an immune reaction in 81% of the people (150) who had never had Covid-19,” the Daily Mail reports. “This, the scientists said, was because they had already been infected with one or more of the common cold coronaviruses known to infect humans.”

About a third of all colds are thought to be caused by coronaviruses that predate the new variant which causes Covid-19.

Uneven distribution

If T-cells are a critical part of the body’s defences against Covid-19, low levels of these “immune hunters” in older people could explain why the elderly are so much more likely to die of the disease.

“Young people and those with mild infections are more likely to have a T-cell response than old people,” say disease experts Franks and Rocklov. “We know that the reservoir of programmable T-cells declines with age.”

And if T-cells are most effective against the new coronavirus when they have already fought off a similar infection, this may have helped some countries such as Japan to contain the pandemic more easily than others.

“Exposure to the related virus that caused the epidemic of severe acute respiratory syndrome (Sars) in 2002-04 might have afforded some protection to East Asians against Covid-19,” says The Observer.

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