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Infection-acquired coronavirus immunity in Chelsea, Massachusetts, wont do much to protect people in Chelsea, Manhattan, let alone Chelsea, London. 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 its spread begins to slow and the number of new cases declines. Becausepeoplewho survive COVID-19likely gain immunity to reinfection (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 is60%-70%, though some theories suggest the needed level couldbelower. How close are we to herd immunity? A Centers for Disease Control and Prevention study measuring seroprevalence, or the level of anti-COVID-19 antibodiesin 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 otherdata. Other locationsshowed levels under 10%, even by late June. Are we close to herd immunity?Thankfully, antibody tests arent everything. The immune system also has memory T and B cells that confer longer lasting immunity, even after antibody levels decline. Evidencesuggests people can have some T cell immunity to COVID-19 from cross reactivity to previous corona viruses theyve fought off, including coronaviruses that cause some forms of the common cold. Thus, lack of COVID-19 antibodies doesnt necessarily mean a lack of immunity, and levels of immunity might be above the levels estimated from antibodyseroprevalence alone. Does this mean we are close to reaching herd immunity, or better yet, might already be there? Unfortunately, no. The key challenge is
