What is herd immunity and how is it related to the novel coronavirus?
When experts at the World Health Organisation (WHO) were recommending social distancing and countries all around the globe were enforcing quarantine measures and shutdowns, the British Government (up until this Monday) had an entirely different approach of dealing with COVID-19.
UK's chief science advisor suggested letting the infection spread among the masses to develop herd immunity in the country. Only those who were suspected of COVID-19 and the elderly were asked to practice self-isolation.
What probably made the government change its policy was a research done by Dr Neil Ferguson, a professor of mathematical biology at the Imperial College of London.
In his study titled 'Impact of non-pharmaceutical interventions (NPIs) to reduce COVID19 mortality and healthcare demand', Ferguson predicted that with their current strategies, US and UK would have at least 2 million and over 500,000 deaths, respectively, from COVID-19.
The study is not published yet but was released on March 16.
As of March 18, UK has confirmed 1,960 cases of the novel coronavirus. Out of these, 71 have resulted in death.
What is herd immunity?
Herd immunity is a concept that says that if a majority of people in a population are exposed (and hence immune) to a microbe, the whole group would become immune - regardless of the presence of some susceptible people. This is because the microbe does not have an easy way to transmit anymore. So, the majority of immune people will, in a way, save the susceptible ones.
Herd immunity also develops through vaccines. The percentage of immune or vaccinated people that are needed to produce herd immunity differ for different diseases. For example, when it comes to measles, which spreads quickly, about 90-95% of people in a population have to be vaccinated for herd immunity to develop. On the other hand, for polio, about 80-85% vaccination would induce herd immunity. This is why most governments recommend compulsory vaccination (for certain diseases) for susceptible groups.
Herd immunity is believed to be one of the possible factors responsible for reducing Zika virus transmission in Brazil during 2015-16.
Suppression, not slowing down
Explaining the extent of the current pandemic, Ferguson wrote in his study that the current scenario is comparable to that of the 1918 flu pandemic where people had no access to vaccines yet. However, we know much more about infectious diseases now. And till a vaccine is developed (which would take at least 12 to 18 months or more), we have two strategies - to slow down the spread of the infection or to suppress it.
The former includes home isolation of infected and at-risk populations and the administration of drugs to affected people to reduce the impact of the epidemic. This method does not include measures to control the transmission of the disease.
The suppression method, on the other hand, aims at reducing the chances of spread of infection by taking stringent regulations (quarantine, social distancing for example) along with administration of drugs to the affected people up until a vaccine is developed.
In a press release by the Imperial College, Ferguson and his team explained that the disease transmission and cases have to be closely monitored if we want to eliminate it. Even if the progression stops, there is a need to be vigilant and reintroduce the suppression strategy quickly if new cases arise.
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Updated Date: Mar 18, 2020 18:00:58 IST
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