Recent Summary on the state of
Antibody Studies
Some excerpts:
Another key question for any sero-survey is how accurate the test was. Tons of antibody tests have hit the market over the past few weeks, and their accuracy is
still being scrutinized. Not all tests have the same degree of accuracy.
Even a test that is very good can give out more false positives than true positives when the prevalence of a disease is very low in a population.
Let’s say you’re running a sero-survey among 1,000 people and only 4% of the population is actually infected. Presume the test correctly identifies positives 100% of the time, meaning it is 100% “sensitive” in scientific parlance.
There are 1,000 people in your sero-survey.
With a 4% infection rate, the test would accurately identify those
40 people who are positive.
But say the test is 95% “specific,” meaning that it returns false positives 5% of the time. Then among the 960 people who are truly negative,
48 people would get a false positive.
You can have more confidence in the signal you’re getting when there’s a higher percentage of the population that’s been infected, as in a situation like New York City, because the number of true positives would drown out a smaller number of false positives, Gelman said. Unfortunately, New York didn’t actually share much information on how accurate its tests were when Gov. Andrew Cuomo first announced the findings of its study on April 23, so the experts I called said they didn’t have much to scrutinize.
In order to achieve herd immunity, scientists say that a community would need to have at least 60% of its population infected. That’s the lowest estimate I’ve been told. Other scientists have told me 80% to 90%. The reason this percentage isn’t precisely known is because it depends on things like exactly how contagious the virus is and also whether people who have been infected are immune forever, or if they lose immunity after a while, which researchers also are furiously working to figure out. None of the studies I’ve seen so far have reported a number anywhere near that high. The highest rate I’ve seen is in Chelsea, Massachusetts, the epicenter of the coronavirus outbreak in that state.
On April 23, Cuomo announced preliminary data from the state’s sero-survey,
saying that 13.9% of state residents had tested positive for antibodies. In New York City, it was about 21%. The state is continuing to test residents in order to generate an ongoing series of “snapshots” of the levels of infection. Cuomo had
updated numbers by April 27 showing huge regional variation.
Kilpatrick, from UC Santa Cruz, said that if the estimates from New York stand up to scrutiny, the infection fatality rate in New York City would be approximately 0.8%.
He told me that is not very surprising, because scientists have been able to get some estimates of infection fatality rates using data from enclosed populations where nearly everyone got tested — on cruise ships. Epidemiologists at the London School of Hygiene and Tropical Medicine, for example,
analyzed data from the Diamond Princess, the ill-fated ship on which more than 700 passengers got infected. Researchers adjusted for the fact that cruise passengers are older than average and estimated the coronavirus’ infection fatality ratio as 0.6%.
The estimates I’ve seen for influenza IFR range from about 0.14% on the upper end to
0.04% on the lower end. So if the IFR for this coronavirus ends up being around 0.5%, that’s still many times worse than the flu.
But that’s
not the main problem. At the end of the day, wherever the coronavirus fatality rate ends up, it doesn’t change the fact that we don’t have any immunity to the virus, which is a critical factor in why we’ve had to behave differently in our response to it.
Marc Lipsitch, head of the Harvard T.H. Chan School of Public Health’s Center for Communicable Disease Dynamics, has estimated that ultimately 20% to 60% of the population could be infected with COVID-19. By comparison, because of immunity provided by flu shots and past infections, only about 10% to 20% of the population gets sick with influenza every year, according to Kilpatrick.
Kilpatrick sketched out what this meant: “If it’s five times deadlier than the seasonal flu, and three times as many people are going to get it, that means we’re going to get 15 times as many deaths. And 15 times 30,000, which is the middle-of-the-road kind of a seasonal flu year, that’s 450,000 deaths — about half a million deaths — that’s a pretty big, scary number, I think.”
As antibody tests become more widely available, there’ll naturally be a temptation to start using the tests for ourselves on an individual basis, to determine if we’re immune and can go about our lives, free of the paranoia and fear that have been plaguing us for the past two months.
But it’s too early for that. Besides the issue of potential false positives, scientists haven’t yet figured out exactly what level of protection an individual has after being infected and whether the protection lasts forever (like with chickenpox) or wanes after a while. The World Health Organization issued a
scientific brief last week warning that detection of antibodies alone shouldn’t serve as a basis for an “immunity passport” allowing an individual to assume they are totally protected from reinfection.