Five Facts about Herd Immunity

“Herd immunity” as the likeliest way to control and ultimately end the coronavirus spread. But this may require many months, during which thousands of people will likely become sick and die, and others will endure long periods of social distancing, perhaps intermittently.

  1. “Herd immunity” to a contagion requires a big majority of a region’s people to become immune to it.

The idea is that so few people remain susceptible the contagion’s spread dies from a lack of new targets. For mumps, a highly contagious disease, 92 percent of a population must become immune to stop its spread. The novel coronavirus is less infectious, so about 70 percent of a population must become immune to choke it off, say researchers at Johns Hopkins University.

  1. Vaccines are crucial to achieving herd immunity.

People become immune to an infectious disease in two ways. They can contract and survive the disease, thus developing natural immunity, at least in most cases. Or they can receive a vaccine, obviously a better option. Absent a vaccine, huge numbers of people must contract the disease to create herd immunity. Researchers worldwide are working toward a coronavirus vaccine, but experts say it probably is 12 to 18 months away.

  1. Intermittent relaxing and tightening of social-distancing rules is likely for many months.

While waiting for a vaccine, thousands of people will probably contract Covid-19 by mingling with others as the U.S. economy and society gradually reopen. If infections rise and hospitals fill, states and counties will be pressed to reinstate variations of stay-at-home or social-distancing rules. “The most likely case,” say experts at Johns Hopkins, is where “infection rates rise and fall over time; we may relax social distancing measures when numbers of infections fall, and then may need to re-implement these measures as numbers increase again. Prolonged effort will be required to prevent major outbreaks until a vaccine is developed.”

  1. On-again, off-again social-distancing orders may continue beyond next year.

A recent study in the journal Science says intermittent periods of social distancing may need to persist into 2022 to keep the U.S. health care system from being overwhelmed with severely ill coronavirus patients. The research, conducted by Harvard University specialists, concludes “it is unlikely that life will return any time soon to the way it was before the virus’ emergence.”

  1. Immunity for Covid-19 survivors seems likely, but not certain.

Experience with viruses suggests that Covid-19 survivors should be immune from reinfection, at least for a time. But researchers have yet to confirm this. If survivors’ immunity proves short-lived or far from universal, it could complicate efforts to re-open society before a widely available vaccine arrives.


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