Herd Immunity: Where Do We Start?

Everyone is anxious to get back to work.

Herd Immunity: Where Do We Start?

Everyone is anxious to get back to work.

Everyone is anxious to get back to work, to restaurants and bars (for me, this includes the many micro brewpubs found in my home city of Seattle), birthday parties, places of worship, etc. To get there, we will need to achieve what is called herd immunity.

What is Herd Immunity?

In a recent opinion piece in the New York Times titled Who Is Immune to the Coronavirus?, Marc Lipsitch (@mlipsitch) — a professor in the Departments of Epidemiology and Immunology and Infectious Diseases at Harvard’s T.H. Chan School of Public Health, where he also directs the Center for Communicable Disease Dynamics — says that “The long-term control of the virus depends on getting a majority of people to become immune, through infection and recovery or through vaccination — how large a majority” is not currently known.

According to a posting on the Johns Hopkins Covid-19 School of Public Health Expert Insights website titled What is Herd Immunity and How Can We Achieve It With COVID-19? , “We will likely need at least 70% of the population to be immune to have herd protection.”

Testing is Key

An important first step in identifying immunity is the performance of serologic tests that detect antibodies that have been made in response to an infection with SARS-CoV-2. A positive test indicates that a person had an immune response to the virus, whether or not the person was symptomatic.

Serologic tests have been submitted to the FDA for Emergency Use Authorization (included here) or, alternatively, the FDA has been notified of their availability per Section IV.D of the FDA guidance (see list). In the latter case, manufacturers may not claim FDA authorization, only that the FDA was notified per Section IV.D; the FDA is currently investigating complaints regarding such false claims. In addition, FDA-notified serologic tests may not be used for diagnosis of or to rule out current infection.

The majority of available serologic tests are ideal for rapid, individual on-site testing. Automated serologic tests that can accommodate large batch sizes, which are of great interest for population screening, are now becoming available.

Home-based testing could speed things up

Home-based testing is currently not allowed under the guidance either for serologic or diagnostic testing. Discussion with the FDA is ongoing regarding possible protocols for telehealth-supervised at-home testing in the event that simple, safe tests become available.

In addition to the FDA website, another publicly available website is the Coronavirus Test Tracker.

In his opinion piece in the New York Times, Dr. Lipsitch emphasizes that decisions about immunity are being made, by necessity, with a minimal amount of data.

Other key points or questions raised in this article include:

  • Studies looking at antibody levels in those who survived previous SARS and MERS infections identify persistence of antibodies for two years or more. By association, this may suggest an immune response and possible protection for those infected with SARS-CoV-2.
  • Assuming immunity is gained after developing antibodies, the chance for herd immunity depends on how many undetected cases there are, which is not currently known. They may range from 10 times the number of detected cases to 100 or even 1,000 times. He emphasizes that these estimates are only statistical inferences.
  • One concern has to do with the possibility of reinfection, especially in those with milder illnesses or who were asymptomatic. Only serologic survey testing can help determine this.
  • “The balance between these uncertainties will become clearer when more serologic surveys, or blood tests for antibodies, are conducted on large numbers of people,” he says.

Almost all experts agree that it is reasonable to assume that a minority of the population in the United States and the world are currently immune to this virus and until we get better testing, knowing when we reach herd immunity will be difficult.

Hayes and TractManager will keep you informed of any progress made regarding testing, treatments, and vaccines.

Author:

David Zieve, MD, MHA

Medical Director, Hayes

David Zieve works closely with the Evidence Solutions analysts and provides clinical oversight.

 

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