Delta Variant Pushes Herd Immunity Threshold Above 80%

According to experts, the fact that the Delta variant spreads easier than the original virus, the proportion of the population that needs to be vaccinated to reach “herd immunity” protection could be 80% and above.

That is a “much higher” bar than previous estimates of 60% to 70%, because delta is twice as transmissible, said Ricardo Franco, an assistant professor at the University of Alabama at Birmingham.

"The problem here is that the Delta variant is … more transmissible than the original virus. That pushes the overall population herd immunity threshold much higher," said Ricardo Franco.

"For Delta, those threshold estimates go well over 80% and may be approaching 90%," he said.

However, herd immunity thresholds of above 80% are not new, measles has a immunity thresholds at 95%.

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What is herd immunity?

Herd immunity is the point at which enough people are immunized that the entire population gains protection.

Historical, we know that herd immunity achieved via vaccination contributed to the eventual eradication of smallpox in 1977. Here is how it works, immune individuals are very unlikely to contribute to disease transmission. So this disrupts the chains of infection, stoping or slowing down the spread of disease.

The greater the ratio of the number of immune individuals to the number of people in the community, the smaller the probability that non-immune individuals will come into contact with an infectious individual.

Herd immunity thresholds (HITs) of some of the well-known infectious diseases.

DiseaseHIT
Measles92–94%
Chickenpox (varicella)90–92%
Mumps90–92%
COVID-19 (Delta variant)80–89%
Rubella83–86%
Polio Fecal–oral80–86%
Pertussis82%
Smallpox71–83%
COVID-19 (Alpha variant)75–80%
COVID-19 (Gamma variant)75–80%
HIV/AIDS50–80%
COVID-19 (ancestral strain)65%
SARS50–75%
Diphtheria62%
Common cold50–67%
Ebola (2014 Ebola outbreak)44%
Influenza (2009 pandemic strain)37%
Influenza (seasonal strains)23%

Sydney Chako

Mathematics, Chemistry and Physics teacher at Sytech Learning Academy. From Junior Secondary School to Tertiary Level Engineering Mathematics and Engineering Science.

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