Could exposure to the coronaviruses that cause the common cold help protect people against Covid-19? Is herd immunity closer than previously thought?
As nations lift lockdowns and experts worry about a potential second peak in cases, our ability to ward off infection is one of the hottest topics of scientific debate.
Ever since it became apparent that children were less vulnerable to Covid-19 early in the pandemic, scientists have speculated that the regular spread of benign viruses in places like schools could have bolstered their immune response to the latest coronavirus.
Now the idea of "cross immunity" among the broader population is gaining some ground.
In a recent post on Twitter, Francois Balloux of University College London noted an "intriguing" lack of an immediate resurgence in Covid-19 cases following the easing of lockdowns in several countries.
Among the possible explanations, he noted, were seasonality and enduring social distancing practices.
But he posited a "wilder" hypothesis as well -- that a "proportion of the population might have pre-existing immunity to #SARSCoV2, potentially due to prior exposure to 'common cold' coronaviruses."
Balloux said that might explain issues like cases where there is no transmission between spouses.
Earlier this month, an American study in the journal Cell suggested between 40% and 60% of the population could be immunized against Covid-19 without ever being exposed to it.
But a team led by Gabriela Gomes of the Liverpool School of Tropical Medicine argues that it is wrong to assume that herd immunity will only be achieved when 60% of people have been infected.
It is more likely, they argue, that the true figure lies between 10% and 20%.
Scientists say if a vaccine was developed it would need 60%-70% coverage to work — but this threshold could be significantly lower for natural immunity.
Herd immunity is a situation in which a population of people is protected from a disease because so many of them are unaffected by it - because they've already had it or have been vaccinated - that it cannot spread.
Immunity among the most socially active people, scientists say, could protect those who come into contact with fewer others.
Britain as a whole is not close to herd immunity, with the government testing surveys suggesting between 5% and 6% of the population have had Covid-19 so far - about three million people.
London, however, has a much higher past infection rate at an estimated 17.5%, so could be approaching a low level of protection.
About six in 10 people living in some of India’s biggest slums have antibodies for the novel coronavirus, indicating they had recovered from infection, in what appears to be one of the highest population immunity levels known worldwide.
Gomes told the Daily Mail: “In idealized scenarios of vaccines delivered at random and individuals mixing at random, herd immunity thresholds are given by a simple formula which, in the case of SARS-CoV-2, suggests that 60%-70% of the population would need to be immunized to halt spread considering estimates of R0 between 2.5 and 3.
“In our model, the herd immunity threshold declines sharply... and remains below 20% for more variable populations.”
The concept of Gomes's reduced herd immunity threshold is based on the groups of the population who are affected.
For example, the coronavirus appears to spread most in cities and among healthcare workers and people who work in jobs that mix with a lot of people, like factory staff or taxi drivers.
They then run the risk of transmitting it to people at a naturally lower risk - someone who lives in a city but travels to visit friends or relatives in the country, for example, or a carer who works in more than one home in a week.
Immunity could also form in places where there are intense local outbreaks.
If workers are immune after outbreaks at food factories, which have happened around the world, that could in turn benefit the families and communities around the planet - even if they haven't personally had the disease.
To cause an outbreak a disease-causing bacteria or virus must have a continuous supply of potential victims who are not immune to it.