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Covid-19: Japan prepares to extend state of emergency nationwide as “untraceable” cases soar

BMJ 2020; 369 doi: (Published 16 April 2020) Cite this as: BMJ 2020;369:m1543

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Covid-19: Herd immunity may be a promising approach to curtail transmission

Dear Editor
The dynamics of disease transmission say that every epidemic curve has a peak, then it flattens and recedes. The flattening and the recession mark the development of herd immunity, which can occur with vaccination or when healthy and non-vulnerable population develops immunity against the disease and this stops further transmission.

If Japan prepares to extend state of emergency nationwide and put total public restriction, it might be successful in decreasing the number of cases and delaying the flattening of the epidemic curve but it does not give a definite solution to the problem. The only solution is developing herd immunity to combat the infection. Normally a herd immunity of 70 to 90% is needed to combat a contagious disease. In biological control, to develop herd immunity, the aim may be to increase transmission rather than decrease it. (1)

R0 or reproductive number is the average number of individuals who will contract a contagious disease from one individual having that disease. (2) In the early outbreak in Wuhan, China, R0 was estimated to be 2.2-2.7. A study from USA has reported R0 of 3.8-8.9. (3) When R0 is less than 1, that is each existing infection causes less than one new infection, then the disease will decline and eventually die out.(2) To bring down R0 to less than one, vaccination or prior infection are the only two alternatives.

Stringent public restrictions may be a welcome initiative if there a vaccine in hand and it is a bargain to buy time to be able to manufacture sufficient vaccines to generate immunity in the population. But as is the case with Covid-19, there is no vaccine in sight and population immunity can be developed only when non-vulnerable individuals develop immunity with prior infection.

As the statistics suggest, the younger population is less susceptible to Covid-19 as compared to the population over 60 years of age. Mortality is 3.6% in individuals 60-69 years, 8.0% in 70-79 years and 14.8% in more than 80 years. (4) And individuals with co-morbidities are more at risk.

Rather than stringent restrictions, if there are selective restrictions such as confining the aged population and those having health issues to their homes, but allowing the others to carry on with their work using basic preventive measures such as hand hygiene and respiratory hygiene, the population may be in a better way prepared to develop herd immunity. And when 80% herd immunity is reached, the R0 and the mortality rates will automatically show a decline and victory over the virus will be sure.

References :

1. Antonovics J. Transmission dynamics: critical questions and challenges. Philos Trans R Soc Lond B Biol Sci. 2017 May 5; 372(1719): 20160087. PMID: 28289255
3. accessed 20th April 2020)
4. (last accessed 20th April 2020)

Competing interests: No competing interests

23 April 2020
Samarjeet Kaur
Assistant Professor, GSVM Medical College Kanpur
GSVM Medical College Kanpur