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Thresholds for Covid-19 herd immunity could be lower

Published Oct 9, 2020


CAPE TOWN- Researchers around the world are now sharpening their focus into herd immunity, how long it could take to be reached, and for how long Covid-19 survivors have immunity against the virus.

Herd immunity is defined as a level of population immunity at which disease spreading will decline and stop after a majority of a population have already been infected and after all preventive measures have been relaxed.

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When 70 percent to 90 percent of the population is immune through natural infection or vaccination, only then is herd immunity achieved, however it might not be uniform across the population.

In an article published on

The Conversation, Senior Lecturer in Mathematics at Stockholm University, Pieter Trapman, said that basic models for Covid-19 suggest herd immunity is achieved when 60 percent of people are immune.

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"This is because in a population where everyone is susceptible to the coronavirus, an infected person is estimated to infect on average an additional 2.5 people. Yet if 60 percent of those theoretical 2.5 people are immune, then only one new infection can take place, and the outbreak cannot grow," he said.

Trapman's researched is based on a very simple model, which assumes that everyone in the population mixes to the same degree and at random. "It’s unrealistic. In our research, we tried to reflect some of the diversity of behaviour found in human populations to show what effect it might have on reaching herd immunity," he said.


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Swedish researchers found that people can develop long-term Covid-19 immunity after infection as T-lymphocytes (T-cells) may have the ability to recognize the virus. Memory T-cells form part of the immune system that responds to a foreign invader and informs B cells about how to craft new virus-targeting antibodies.

As more people recover from the virus and are immune from it — albeit for a certain period of time — the closer a population is to herd immunity. This means that herd immunity could be reached at a lower human cost than previously expected.

Trapman said that his study's findings should be interpreted only as a demonstration of how differences in behaviour can affect herd immunity.

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"These figures aren’t exact values, or even best estimates. The activity levels and contact rates between age groups that we used in the model were simply illustrative," he said.

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