By MICHEL ACCAD, MD
With cases of COVID-19 either disappeared or rapidly diminishing from places like Wuhan, Italy, New York, and Sweden, many voices are speculating that herd immunity may have been reached in those areas and that it may be at hand in the remaining parts of the world that are still struggling with the pandemic. Lockdowns should end—or may not have been needed to begin with, they conclude. Adding plausibility to their speculation is the discovery of biological evidence suggesting that prior exposure to other coronaviruses may confer some degree of immunity against SARS-CoV2, an immunity not apparent on the basis of antibody seroprevalence studies.
Opposing those viewpoints are those who dismiss the recent immunological claims and insist that rates of infections are far below those expected to confer immunity on a community. They believe that the main reason for the declining numbers are the behavioral changes that have occurred either under force of government edict or, in the case of Sweden, more voluntarily. What’s more, they remind us that the Spanish flu pandemic of 1918-1919 occurred in 3 distinct waves. In the summer of 1918 influenza seemed overcome until a second wave hit in the fall. Herd immunity could not possibly have accounted for the end of the first wave.
The alarmists may have a point. However, recent history offers a more instructive example.
Until early 2015, epidemiologists considered Mongolia to be exemplary in how it kept measles under control. In the mid-1990s, the country instituted a robust vaccination program with low incidences of outbreaks, even by the standards of developed countries. In the early 2000s, it adopted a 2-step MMR immunization schedule and, after 2005, its vaccination rates were upwards of 95%. From 2011 through 2014, not a single case of the virus was recorded, leading the WHO to declare measles “eradicated” from Mongolia in November