Covid-19: Herd immunity can be achieved at 43%, new study finds
A region can achieve herd immunity against Covid-19 with fewer people than previously estimated, a new study said on Wednesday, but experts cautioned that the humanitarian and medical costs of taking the route continue to remain high, especially in India.
The study, published in the journal Science and conducted by mathematicians from the University of Nottingham and University of Stockholm, categorized people into separate groups reflecting their age and social activity levels.
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Their model found that the threshold percentage of population who need to be infected for the chain of transmission to be broken – known as herd immunity – was 43% and not 60%, as thought earlier.
“It is a positive result that herd immunity appears earlier, but also that immunity prior to herd immunity has a bigger effect on reducing disease spread,” said Tom Britton of Stockholm University and the lead author of the paper.
But the scientists warned that the 43% figure should not be interpreted literally as an exact value and instead should be seen as a study of how population differences affect herd immunity.
“Our findings have potential consequences for the current Covid-19 pandemic and the release of lockdown and suggests that individual variation (for example in activity level) is an important feature to include in models that guide policy,” said Professor Frank Ball from the University of Nottingham, one of the authors of the paper.
The study devised six age categories and three activity categories: high, medium and low; and assumed that within each age bracket, 50% have normal activity, and 25% each have low and high activity level. The basic reproduction number, or the number of people infected by one patient, was assumed at 2.5
The scientists flagged two important things.
One, that activity level mattered more than age differences and that more socially active individuals were found more likely to get infected and spread the virus.
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The second was that herd immunity can be achieved in two ways: by vaccinating enough number of people or if enough people are infected and develop immunity against the disease. “The herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination,” Bell said.
The study also said the previous figure of 60% assumed that each individual in the population was equally likely to be vaccinated, and hence immune. However, that was not the case if immunity arises as a result of disease spreading in a population consisting of people with many different behaviours. This is why the herd immunity threshold was lower in a heterogeneous model.
The debate around herd immunity, which happens when so many people in a community become immune to an infectious disease that it stops the disease from spreading, is at least six months old. Countries such as the United Kingdom backed off from the herd immunity strategy after studies predicted millions of deaths.
While publishing the study, Science editor-in-chief Holden Thorp noted that even if the model’s prediction was correct, none of the seroprevalence studies suggested that any country was close to achieving herd immunity. “Continuing non-pharmaceutical interventions around the world is still of great importance,” he said.
Other experts pointed to a recent study that showed levels of an antibody found in recovered Covid-19 patients fell sharply in 2-3 months after infection for both symptomatic and asymptomatic patients – which means immunity to the virus may not last.
“So, even if herd immunity was achieved, there is no guarantee that antibody levels will stay the same,” said V Ramana Dhara, professor at the Indian Institute of Public Health, Hyderabad.
“The immunity depends on whether protective antibodies are produced. Both the CDC and WHO are stating there is yet no evidence of this so it should not be assumed that Covid antibodies are protective. The authors’ model has assumed immunity for an extended period but if that’s not the case, other models are needed. If the herd immunity does turn out to be protective, then the 43% makes it more feasible, but that’s a big if right now,” he added.
Snehal Shekatkar, a professor at the Centre for Modelling and Simulation, Savitribai Phule Pune University, said the study’s prediction should be taken with a large pinch of salt since the study uses compartmental models of epidemiology instead of more accurate contact-network based models.
“Adding heterogeneity in the population in the form of age-structure based contact rates certainly improves compartmental models but they nevertheless lack many important structural features present in the real-world contact networks. Also it is important to note that a particular value for R0 has been used in the study (R0=2.5), and for different R0 values, the threshold would be different,” he added.
Shekatkar said achieving herd immunity was not feasible unless a region was very sparsely populated. “Even .1% of the population is a very high number of people to get infected or die,” he said.