WHO herd immunity vaccine, Infection-acquired COVID immunity in Chelsea

 WHO herd immunity vaccine, Infection-acquired COVID immunity in Chelsea.

As the recent surge of COVID-19 cases slowly begins to decline, some have theorized that herd immunity might have already kicked in, meaning the worst could be over. We hope these theories prove true, but we fear they will not.

Herd immunity refers to enough people in a population having immunity to a disease that spread begins to slow so that new cases are smaller during each period. Since people who survive COVID-19 likely gain immunity to re-infection (even if temporarily), it becomes increasingly difficult for the virus to spread as more and more people are immune. The consensus estimate for the COVID-19 herd immunity threshold is around 60%-70%, though some theories suggest the needed level could be lower. 

How close are we to herd immunity? A Centers for Disease Control and Prevention study measuring seroprevalence, or the level of anti-COVID-19 antibodies in different areas, showed that, unsurprisingly, the highest levels were in New York City, where just above 20% had COVID-19 antibodies as of early May, an estimate that matches other data. Other locations showed levels under 10%, even by late June.

Are we close to herd immunity?

Thankfully, antibody tests aren’t everything. The immune system also has memory T and B cells that confer longer lasting immunity, even after antibody levels decline. Evidence suggests people can have some T cell immunity to COVID-19 from cross-reactivity to previous corona viruses they’ve fought off, including corona viruses that cause some forms of the common cold. Thus, lack of COVID-19 antibodies doesn’t necessarily mean a lack of COVID-19 immunity, and levels of COVID-19 immunity might be above the levels estimated from antibody seroprevalence alone.

Does this mean we are close to reaching herd immunity, or better yet, might already be there? Unfortunately, no. The key challenge is extensive local variation. For instance, a CDC study showed that, while seroprevalence levels in New York City were 20%, levels in neighboring areas of Connecticut were four-fold lower, at only 5%.

Variation within New York is just as dramatic. In mid-April, the New York State Department of Health found that while New York City had a seroprevalence of 23%, neighboring Westchester and Rockland Counties were 16%, with Long Island at 13%, and the rest of New York a mere 4%. 

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