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10 June 2020updated 06 Oct 2020 9:45am

Why the search for mysterious Covid-19 “dark matter” will not lead to enlightenment

The term is an eye-catching shorthand for speculation that there may be pre-existing immunity to Covid-19.

By Phil Whitaker

There has been increasing discussion recently of a putative immunological “dark matter” that might explain certain observations emerging from the Covid-19 pandemic. Until now, it has been assumed that the varying successes between countries in suppressing the disease have resulted from differing approaches to infection control. Strict immigration management and high levels of testing, tracing and isolating (TTI), as seen in the Far East: good. Low levels of same, as seen in many Western countries, particularly the UK: bad.

The importance of TTI in controlling outbreaks of communicable diseases is long established in public health, but globally there are some interesting outliers. Japan, for example, has maintained a very low death rate with far less aggressive TTI and lockdown measures than many of its neighbours. And some analyses have suggested that in many countries rates of infection peaked several days prior to lockdowns being introduced, implying that some as-yet-unknown factor was already beginning to limit the spread of the infection. Lastly, the unhappy laboratory represented by the cruise ship Diamond Princess – in which 3,700 people were quarantined in a closed community – has been cited as further evidence of some hidden inhibition on Covid-19. Up to a third of the passengers and crew are estimated to have been infected, yet only 200 displayed clinically significant symptoms, nine of whom died. What protected the other 1,000 – who had contracted the virus without even knowing it?

The term “dark matter” is an eye-catching shorthand for speculation being fuelled by interesting results coming from labs in the US, Germany and Singapore. These all point to a substantial level of pre-existing immunity to Covid-19 among people who have never had the infection. It appears that past exposure to other, trivial coronavirus infections (such as the common cold) endows many individuals with the ability to clear Covid-19 with minimal or no symptoms. This may also explain children’s notably low rates of clinical disease. Nurseries and schools act as virus exchange centres. Frequent exposure to other infections with antigenic similarity to Covid-19 may mean young people’s immune systems are turbocharged to repel this novel threat.

The “dark matter” idea is causing much optimism in some quarters. Remember the concept of “herd immunity”? Essentially, the higher the proportion of the population that is resistant, the less chance an infection has of spreading because it becomes increasingly unlikely that the virus will encounter fresh susceptible hosts. Achieve immunity levels of 60 per cent and Covid-19 should start to be snuffed out.

Initially, modelling of the pandemic assumed no immunity in the population: were that to be the case, the tens of millions of infections needed to achieve herd immunity would be unconscionable given an infection fatality rate (IFR) of 0.5 to 1 per cent. But if a substantial bulk of the public has cross-immunity, we may already be near the point at which Covid-19 will lose its sting.

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In science, however, one has to guard against confirmation bias – the tendency selectively to attach undue weight to evidence that seems to support one’s point of view. The Diamond Princess, for instance, merely confirms global experience of Covid-19: an IFR close to 1 per cent, and a clinically significant disease burden of around 20 per cent. That two-thirds of passengers escaped infection appears closely linked to the imposition of lockdown on board. A second wave was seen among crew who continued to have interpersonal contact with passengers – analogous to the situation for health and social care workers in wider society.

What of the stark differences in death rates between Far Eastern and Western nations? Of course, it is possible that other coronaviruses with antigenic cross-reactivity with Covid-19 are more prevalent in these countries, endowing their populations with more of the mysterious “dark matter”. But that is to ignore well-documented demographic differences with the West, where we have vastly greater numbers of people with the risk factors that make Covid-19 so deadly. And the marked variation in successful control of the pandemic between different European nations correlates strongly with the degree to which they have emulated the infection control practices of Far Eastern nations. It seems implausible that countries such as Denmark, Norway, Germany and Austria are endowed with immunological “dark matter” absent in France, Italy, Belgium and the UK.

What of the finding that peaks of infection preceded formal lockdowns by several days? The UK illustrates the most plausible explanation: society at large began to implement its own voluntary lockdown measures well before the government finally decided to impose them. Anticipatory behavioural change seems more likely than the workings of some hidden immunological protection.

The lab studies demonstrating substantial pre-existing immunity against Covid-19 merely provide the explanation for what we know from experience: that this disease causes a spectrum of illness, with the great majority either asymptomatic or only trivially indisposed. The results from these experiments will be vital in helping to assess the various vaccine candidates currently under development. And the studies provide a platform from which to begin to understand the variations in immune response between mildly affected patients and those who are gravely sick that will, in time, better inform treatment for the worst cases. But they don’t hold any hidden secrets that might get us out of lockdown any more easily – no matter how much some might hope or believe they do. 

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This article appears in the 10 Jun 2020 issue of the New Statesman, A world in revolt