Getting Technical
When herd immunity has been attained across a population it is still possible to have large outbreaks particularly in areas where vaccination rates are low . “ We ’ ve seen that play out in certain countries where misinformation about vaccine safety has spread ,” the epidemiologist Salathé says . “ In local pockets , you start to see a drop-in vaccination , and then you can have local outbreaks which can be very large , even though technically the level for herd immunity had been reached according to the calculations .” The practical goal is to prevent people from becoming unwell rather than to attain a theoretical number in a model .
The other related question is " How high is the threshold for SARS-CoV-2 ?"
Reaching herd immunity depends to a greater or lesser extent on what is actually happening in the population . Kwok and his team have estimated the R t in more than 30 countries , using data on the daily number of new Covid-19 cases as from March 2020 . They then used these values to calculate a threshold for herd immunity in the population of each country .
The numbers ranged from as high as 85 % in Bahrain , where the R t had been determined at 6.64 , to as low as 5.66 % in Kuwait , where the R t was measured at 1.06 . Kuwait ’ s low numbers showed how it was putting in place effective measures to control the virus , such as establishing local curfews and banning commercial flights from many countries . If the country stopped those measures , Kwok says , the herd-immunity threshold would go way up .
Herd-immunity calculations such as the ones in Kwok ’ s example are built on assumptions that might not reflect real life according to Samuel Scarpino , a network scientist studying infectious disease at North-eastern University in Boston , Massachusetts . “ Most of the herd-immunity calculations do not have anything at all to say about behaviour . They assume there are no interventions or changes in behaviour or anything like that ,” he says . This means that if a change in people ’ s behaviour ( such as physical distancing ) drives the R t down , then “ as soon as that behaviour goes back to normal , the herd-immunity threshold will follow that change ”.
Estimates of the threshold for SARS-CoV-2 range from 10 % to 70 %, or even more but models that calculate numbers at the lower end of that range rely on assumptions about how people interact in social networks that might not hold true , According to Scarpino . low-end estimates assume that people with many contacts will get infected first , and because they have a large number of contacts , they will spread the virus to more people .
Scarpino refers to these people as ‘ super spreaders ’. He goes on to say that as more people gain immunity to the virus , the transmission chains among those who are still susceptible are greatly reduced and therefore “ as a result of that , you very quickly get to the herd-immunity threshold ”. However , these types of situations are seldom simple and straightforward . If it turns out that anybody could become a super spreader , then “ those assumptions that people are relying on to get the estimates down to around 20 % or 30 % are just not accurate ,” Scarpino explains .
The result is that the herd-immunity threshold will be closer to 60 – 70 %, which is closer to what has generally been found to occur . RACA
www . hvacronline . co . za RACA Journal I July 2021 37