Getting Technical
CHARLES NICOLSON
Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to good use by applying speciality chemicals in mining and industrial processes where water is a major factor . This created an enduring interest in water technology , a passion that expanded to the HVAC industry in 1984 when he joined BHT Water Treatment . Since then , water technology in HVAC water circuits has continued to be an abiding interest .
QUANTITATIVE ASPECTS IN HERD IMMUNITY
By Charles Nicolson
My previous article was a qualitative description of infections due to coronaviruses and described one of the methods used for creating herd immunity .
The method I discussed was simply to let the coronavirus run its course and allow people to return to their normal daily living while implementing precautions to protect those at higher risk levels . Specialist epidemiologists no longer even consider this type of approach because it has led to unacceptably high loss of life without speeding up the return of society to normal . Another document described in a recent ‘ Getting Technical ’ article was the Great Barrington Declaration which recommends that people at lower risk of severe Covid-19 return to normal life thereby allowing SARS-CoV-2 to spread to a sufficient level to give Herd Immunity .
People at high risk , such as elderly people , it says , could be protected through measures that are largely unspecified . The writers of the declaration received an audience in the White House and sparked a counter memorandum from another group of scientists in The Lancet , which called the herd immunity approach a “ dangerous fallacy unsupported by scientific evidence .”
Public-health experts do not support herd immunity being used when vaccines are not available . Marcel Salathé , an epidemiologist at the Swiss Federal Institute of Technology in Lausanne says , “ I ’ m a bit puzzled that it ’ s now used to mean how many people need to get infected before this thing stops .”
Epidemiologists can calculate the approximate percentage of a population required for herd immunity to arise . This percentage depends on a reproduction number designated as R0 , which is the number of cases likely to result from one infected individual in a normal , well-mixed population , according to Kin On Kwok , an infectious-disease epidemiologist and mathematical modeller at the University of Hong Kong .
The herd-immunity threshold is 1 – 1 / R , therefore , as more people become infected by each individual who has the virus , the
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higher the percentage of the population needs to be immune to achieve herd immunity .
Looking at an example of a more commonly known disease , measles , which is highly infectious to normal well-mixed populations , the R0 is between 12 and 18 which calculates to a herdimmunity threshold of over 90 % of the population .
The R0 assumes that everyone is susceptible to the virus . However , a variation of R0 called the R effective ( abbreviated Rt or Re ) is more commonly used in calculations because it allows for changes in susceptibility within the population being considered .
An illustration depicting British Prime Minister , Boris Johnson , presenting R .
The formula 1 – 1 / R0 produces a theoretical number for herd immunity but this is not a specifically defined point but rather a factor similar to a gradient , according to Gypsyamber D ’ Souza , an epidemiologist at Johns Hopkins University in Baltimore , Maryland , and , because variables can change ( including R ) as well as the number of people susceptible to a virus ; herd immunity is not a steady state .
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RACA Journal I July 2021 www . hvacronline . co . za