[Rushtalk] What the first coronavirus antibody testing surveys can tell us

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Thu May 21 12:53:42 MDT 2020

What the first coronavirus antibody testing surveys can tell us

Health | Analysis 24 April 2020 
By Michael Le Page 

Testing for antibodies is Cisliano, Italy
Testing for antibodies is Cisliano, Italy

Nicola Marfisi/AGF/Universal Images Group via Getty Images

A few initial surveys looking at how many people have antibodies against
the coronavirus have suggested that far more people have been infected
than previously thought. But we need to be very cautious about these
preliminary results.

So far, almost all testing has been looking for the presence of the
virus in swabs of the nose or throat. But not everyone suspected to be
infected gets tested in this way, so we know the actual number of cases
in any country or region must be higher than the official figure.

The question is, how much higher? These figures matter because they can
help indicate the best strategy for removing lockdowns or social
distancing measures. This is why initial studies using a different kind
of testing have been hitting the headlines. Unlike swab tests, antibody
tests look to see how many people have antibodies against the
coronavirus in their blood – a sign of past infection. 

For instance, a study at Zhongnan Hospital in Wuhan, China, found that 2
per cent of 3600 staff there had antibodies to the virus. That is
surprisingly low, given the scale of the outbreak in Wuhan and that
hospital staff are probably more likely to get infected than the general

By contrast, a study in Santa Clara County, California, where just 50
deaths have been attributed to the virus so far, claimed up to 4 per
cent of people there had already been infected – up to 85 times the
official figure – based on a survey of 3000 people. Based on this
finding, the team estimated that less than 0.2 per cent of people
infected with the coronavirus die, which is far lower than most other
estimates, which tend to fall somewhere between 0.7 and 3.4 per cent.

Read more: Can you catch the coronavirus twice? We don’t know yet
But this study, which has not yet been peer reviewed, and similar work
elsewhere have come under fire. “We should not be making policies based
on press releases until the studies are properly reviewed,” says Daniel
Larremore at the University of Colorado, Boulder. “It really matters
that we get these things right.”

There are two main issues. The first is that when only a tiny percentage
of a population has been infected, antibody testing can wildly
overestimate the number of people who’ve encountered the virus. The
second is that it is hard to test a representative sample of the
population, so results can end up being greatly skewed.

“Mathematically, it’s a problem when the prevalence is very low,” says
Merit Melin of the Finnish Institute for Health and Welfare.

This is because, in these situations, the number of people wrongly
identified as having had the coronavirus – false positives – can be
higher than the number of people who really have been infected.

For instance, suppose an antibody test has specificity of 99 per cent –
that is, just 1 in 100 results is a false positive. If 50 per cent of a
population has been infected, this test might tell you that 51 per cent
have antibodies – pretty close. But if only 0.1 per cent have been
infected, the test will tell you that about 1 per cent have been
infected – a tenfold overestimate.

Researchers can correct for this, and the Santa Clara results were
adjusted to take this into account. However, this adjustment was based
on the test having a specificity of 99.5 per cent. But if the true test
specificity is just slightly lower at 98.5 per cent, for example, all
the reported positives would be false positives.

Another issue is how you recruit volunteers for antibody testing. The
Santa Clara study found volunteers through Facebook, and it’s possible
that people who suspect they have had covid-19 may have been much more
likely to volunteer. Ideally, studies should randomly select people.

Better studies should give us a clearer idea of the true numbers.
Melin’s team, for instance, is using a second test to confirm all
positives, with results for Finland expected at the end of the month.

New Scientist contacted the lead author of the Santa Clara study, but he
was not available for comment.

Article amended on 27 April 2020 


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