herd immunity.

Does Covid-19 Infection Confer Immunity?

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The immune system can retain a memory of infection long after a person recovers from a viral illness.  Immune cells and proteins that circulate in the body can identify and kill the virus if it is encountered again.  

Long-term immunity depends upon several components:

  • Antibodies, circulating proteins in the blood, recognize viruses and destroy them.

  • Killer T cells, a type of lymphocyte, recognize and kill pathogens including viruses.

  • B cells make new antibodies when needed.

A recent study by Washington University researchers, published in the journal Nature, suggested that mild cases of Covid-19 leave lasting antibody protection and that repeated infections of Covid-19 are unlikely.  B cells multiply rapidly during infection, but, as patients recover, most B cells die off and blood antibody levels drop.  Some B cells, known as long-lived plasma cells, remain in the bone marrow, secreting antibodies, and are ready to respond against a new exposure.  These plasma cells will likely confer lasting immunity.

In a recent study at the National Institutes of Health that analyzed Covid-19 cases of varying severity, 95% of participants had 3 or more of the 5 immune-system components studied when measured for up to 8 months after infection.

Based upon these results, it appears that Covid-19 infection does confer lasting immunity in most infected individuals.  Both groups of researchers agree that it remains to be seen if vaccination also induces the immune responses needed to provide long-term immunity.  Vaccine-related studies are ongoing.

Why Herd Immunity Alone Is NOT a Viable Option for Eradicating COVID-19

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As discussed in an earlier blog, herd immunity occurs when enough people become immune to an infectious disease, either through infection or a vaccine.   A virus will die out once uninfected individuals are rarely in contact with those that are contagious.  But achieving herd immunity by natural transmission and without a vaccine is gained at the cost of many lives.

In August, the World Health Organization’s Michael Ryan warned journalists, “We are nowhere close to the levels of immunity required to stop this disease transmitting.  We need to focus on what we can actually do now to suppress transmission and not live in hope of herd immunity being our salvation.”

Experts believe that 60-70% of Americans will need to become immune before our country reaches herd immunity.  But, according to Dr. Dale Bratzler, OU Medicine enterprise chief quality officer, “Getting to herd immunity through infection is not the direction Oklahoma wants to go right now.”   To get there, nearly 2.4 million Oklahomans would have to contract COVID-19.  Using the lowest available estimate of case fatality rate of 0.3%, this would mean more than 7,000 Oklahomans (more than ten times the current total) would die, while using the state’s current 1.27% death rate means that more than 30,000 people would die.

It is believed that, even in the absence of a vaccine, the Brazilian city of Manaus has reached herd immunity.  As of August, researchers estimated that 66% of the population had been infected.  In that city of over 2 million people, 4,000 people have died so far, a high death toll for a city where only 6% of the population is over 60.  This experience reveals “that an unmitigated outbreak will lead to very significant levels of illness and mortality,” says Bill Hanage, an epidemiologist at Harvard University, “which is what we’ve been saying since February.”

So, the consensus opinion of experts, with which I also concur, is that we do need to achieve herd immunity against COVID-19 through a combination of natural immunity due to infection AND an aggressive vaccine program.  And, while awaiting the arrival of COVID-19 vaccines, we all need to continue to wear masks, wash our hands, avoid touching our faces and maintain social distancing when appropriate.  Frequent testing and contact tracing are also needed for those who have been exposed to COVID-19.  If we do these things we will save thousands if not millions of lives.

For more information check these links

https://www.thelancet.com/journals/lancet/article/PIIS0140-67362031924-3/fulltext

https://www.inquirer.com/health/coronavirus/covid-coronavirus-herd-immunity-vaccine-20200924.html