Fact sheet: Understanding Covid-19 herd immunity

The Claim: The government announced last week that it has since procured 13 million Covid-19 vaccines out of the 20 million required in order to achieve herd immunity. The country seeks to realise herd immunity by vaccinating 10 million Zimbabweans by the end of the year.
The Verdict: claim verdict

The government announced last week that it has since procured 13 million Covid-19 vaccines out of the 20 million required in order to achieve herd immunity. The country seeks to realise herd immunity by vaccinating 10 million Zimbabweans by the end of the year.

What is herd immunity?

·         According to the World Health Organisation (WHO), herd immunity, also known as population immunity, is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.

How is herd immunity against Covid-19 realised?

·         Herd immunity against Covid-19 should be achieved by protecting people through vaccination, not by exposing them to the pathogen that causes the disease.

·         Vaccines train people’s immune systems to create proteins that fight disease, known as ‘antibodies’, just as would happen when individuals are exposed to a disease but – crucially – vaccines work without making them sick.

·         Inoculated people are protected from getting the disease in question and passing on the pathogen, breaking any chains of transmission.

·         To safely achieve herd immunity against Covid-19, a substantial proportion of a population should have been vaccinated, lowering the overall amount of virus able to spread in the whole population. Once herd immunity has been achieved, vulnerable groups who cannot get vaccinated due to health conditions like allergic reactions to the vaccine are kept safe and protected from the disease.

What percentage of the population should be vaccinated to achieve herd immunity?

·         The percentage of people who need to be immune in order to achieve herd immunity varies with each disease.

·         According to WHO, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%.

·         However the proportion of the population that must be vaccinated against COVID-19 to begin inducing herd immunity is not known.

·         This is an important area of research and will likely vary according to the community, the vaccine, the populations prioritized for vaccination, and other factors.

·         However, the estimated threshold is at least 65%-70%.

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