USA TODAY International Edition
Don’t count on COVID herd immunity just yet
The best way to get there without massive loss of life is a vaccine
As the surge of COVID- 19 cases slowly declines, 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. Because people who survive COVID- 19 likely 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 is 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.
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 coronaviruses they’ve fought off, including coronaviruses that cause some forms of the common cold.
Thus, lack of COVID- 19 antibodies doesn’t necessarily mean a lack of immunity, and levels of immunity might be above the levels estimated from antibody seroprevalence alone.
Variations from town to town
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, the CDC study showed that while seroprevalence levels in New York City were 20%, levels in neighboring areas of Connecticut were fourfold lower, at only 5%.
Variation within a state 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 3.6%.
In mid- May, the Boston Public Health Commission, in collaboration with Massachusetts General Hospital, found analogous variation across different Boston ZIP codes: Positive tests for antibodies ranged from 6.3% in part of Dorchester to 13.3% in East Boston.
A small study, also from Massachusetts General Hospital, found that in Chelsea, across the Mystic River from Boston, 32% of residents tested positive for antibodies.
These patterns are not unique to New York or Massachusetts. And though T and B cell immunity means overall immunity rates could be higher than suggested by antibodies alone, we expect that the presence of T and B cell immunity will vary just as antibody presence does.
Since we’ve seen how just one person with COVID- 19 can restart the cycle of exponential growth, we can’t get complacent from knowing that a few places might be close to herd immunity. Their neighbors likely aren’t, which means they aren’t safe from new flareups, either. From California and Louisiana to Hong Kong and France, relaxation of and reduced compliance with safety measures have led to new waves of cases.
Achieving broad immunity
The best way to achieve herd immunity without massive loss of life is with a vaccine that is both highly effective and taken by the vast majority of individuals, so that all regions have at least 60%- 70% of population with true immunity. This won’t be easy. While we have multiple vaccine candidates in late- stage clinical trials, we know vaccines are hard to make. And we know some people fear taking vaccines.
To maximize levels of immunity nationwide and to reopen the American economy as soon as possible, we would do well to focus on making any future vaccine widely accessible, and ideally, free of charge. Public confidence in the vaccine can be boosted by a Food and Drug Administration approval process that is science- based, rigorous and transparent.
Whether from antibodies or T cells or both, infection- acquired immunity in Chelsea, Massachusetts, won’t do much to protect people in Chelsea, Manhattan, let alone people in Chelsea, London. To achieve herd immunity, we will all need to get our shots.
Pranam Dey is a medical student at Yale University. Elizabeth H. Bradley is president of Vassar College and a professor of science, technology and society. Dr. Howard P. Forman is a professor of public health policy, management, economics and radiology at Yale.