Coronavirus Herd Immunity: How Trustworthy Are Recent Antibody Tests?

People have been catching and transmitting the deadly coronavirus at much higher rates than previously believed.
By on 04/23/2020 2:00 PM

Source: MedicineNet Health News

From California to New York, growing clusters of news headlines report unusually high COVID-19 immunity rates in American communities, suggesting people have been catching and transmitting the deadly coronavirus at much higher rates than previously believed. Meanwhile these tests suggest the death rate of the novel coronavirus is much lower than previous estimates.

Any increase in the rate of people immune to COVID-19 is good news. It's a sign we can reopen sooner.

The promise of a rapid antibody test, if realized, would mean people who had recovered from the novel coronavirus could return to some normal activities, and the country could begin to reopen.

But because coronavirus is so new and still relatively rare in the population, extrapolating immunity rates from flawed tests and tiny samples may give us an inaccurate view of who and how many are really immune.

Estimating Herd Immunity

When enough people in a community are immune to a virus, the rest who aren't will have enough protection from infection that they no longer need to worry about becoming sick. That’s herd immunity, according to epidemiologist Caroline L. Trotter, Ph.D., for Medscape.

But the percentage of people who need to be immune depends on the disease. Specifically, it depends on how contagious a disease is, according to 2015 Princeton abstract.

Measles, one of the most contagious diseases, requires at least 92% of a population to be immune to prevent its spread, according to researchers.

SARS-CoV-2, the virus that causes COVID-19, is not as contagious as measles. However, it is a virus that spreads relatively quickly.

That is why at least 70% of a population must be immune to achieve herd immunity from COVID-19, according to health professors for the Johns Hopkins Bloomberg School of Public Health.

Antibody Tests Show Unusually High Infection Rates

Many recent antibody studies suggest a higher-than-expected infection rate in the United States.

A study in Santa Clara County, California—the first of its kind in the US according to Stanford University—found that anywhere from 2.5% to 4.16% of the population had tested positive for COVID-19 related antibodies. If that is true, the corresponding fatality rate for COVID-19 in Santa Clara County would be .12% to .2%, explains Medscape, which is far below most estimates.

One biostatistician estimated the actual infection rate in Santa Clara at between 1% and 2%, though, because of several flaws in the Stanford research. That would keep fatality rate estimates in line with most others.

Another recent study sponsored by the state of New York found similarly unusual, high rates of infection among 3,000 people studied.

But before results were in, primary care physician Dr. Ayman Attia warned the CBS news affiliate in New York that these tests can be flawed.

How Antibody Tests Fail: False Positives

To understand what might be causing these discrepancies, it is important to understand what a "false positive" is. MedicineNet medical author , explains.

He says that when a test shows that you have a condition, but you actually do not, your test has tallied a false positive.

If a COVID-19 test provides many false positives, this obscures the true number of infected people in a community. It also skews the fatality rate of the disease.

Even the most accurate serological tests for any disease will return a certain percentage of false positives. When the number of people receiving the test and the accompanying results are small, those few false positives can skew the data much more than when testing for a more common condition with a larger number of infected people.

In other words, even well-designed antibody tests will produce some "noise" in the data, with some tests returning positive results when the person tested has no immunity. And when only a small percentage of a population has an infection, the actual number is more easily obscured by the noise of false-positive results.

To make matters more difficult, assays for COVID-19 have been difficult to produce quickly and accurately.

Additional difficulties include tests that may show up positive for another, milder coronavirus type that causes only common cold symptoms, Dr. Attia said. And nobody knows how long a person who recovers remains immune to COVID-19.

Tests Still Needed, Despite Flaws

Although current research remains hampered by the above problems, scientists generally agree that such tests are necessary.

Testing the population quickly and accurately will speed up the reopening of the country. It could allow some businesses to reopen, and some people to return to public life.

But although preliminary efforts are encouraging, it is important to keep things in perspective, A. Marm Kilpatrick, PhD, an infectious disease expert zoologist for UC Santa Cruz, stated on Twitter that the Santa Clara study was badly flawed.

"It's a serological study, which is fantastic," he said. "We need these kinds of studies and data badly. Unfortunately this paper is badly misleading."

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