Why vaccines are essential to herd immunity2 min read . Updated: 21 Sep 2020, 09:07 AM IST
Scientists say 60% to 70% of the population would have to gain immunity to Covid-19 through infection or vaccination to reach widespread protection
An infectious disease like coronavirus, if left unchecked, can spread rapidly through a community. Scientists use the transmission rate, or R0 number, to denote how many people a typical carrier of a disease may go on to infect.
In the absence of control measures such as social distancing or mask wearing, the novel coronavirus has an R0 number of between 2.5 and 3.0, meaning the average carrier would infect between two and three people.
As those people go on to infect others, the disease can spread rapidly through a community that hasn’t been exposed to it before.
However, as more people contract the disease and gain some level of immunity, the disease’s spread will slow naturally.
When enough people become immune such that the whole community is protected, it’s called herd immunity. Herd immunity can sometimes occur naturally from survivors of the disease within a population, but often not without many deaths. Covid-19 has so far killed close to 200,000 people in the U.S. Epidemiologists believe only a small percentage of the nation has been infected and developed some level of immunity.
The introduction of a vaccine can be the quickest, safest way of creating herd immunity, since people can develop immunity without getting the disease.
Scientists estimate that 60% to 70% of the population might need to gain immunity to Covid-19 before herd immunity is reached. That’s based on an estimated R0 number of 2.5 to 3. Compare that with measles, a highly contagious pathogen with an R0 number of 12 to 18 and a 90%-to-95% herd threshold.
Depending on how effective a vaccine is, it might not provide complete immunity. In that case, more people might need to be vaccinated to reach herd immunity.
In New York City, an epicenter of the U.S. epidemic, some researchers estimate 25% of the population has protective antibodies against Covid-19. In other parts of the country, the rate could be far lower. Waiting for 60% of the global population to gain natural immunity could mean billions of infections and millions of deaths.
Vaccines work by exposing people to a disabled version, or component, of a virus. This teaches the immune system to recognize the virus, so it can later defend against it in the event of an actual infection.
About 170 Covid-19 vaccines are in development around the world, according to the World Health Organization, each one promising to protect people from the deadly coronavirus and allow them to go back to work and school. A handful are starting or nearing the final stage of testing.
The testing trials could provide some results as soon as the end of October. The U.S. government plans to begin shipping any Covid-19 vaccines within 24 hours of a green light from regulators. Health officials including Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, have said that the vaccine likely won’t be widely available until later in the year.
Write to Alberto Cervantes at Alberto.Cervantes@wsj.com