With efforts to combat the Covid-19 pandemic having resulted in the imposition of lockdown in about a third of the world’s countries, the debate over the long journey back to recovery has already begun.
As parts of the world consider reopening their economies, one of the options experts are talking about is herd immunity.
The Global Vaccine Alliance (Gavi) defines herd immunity as the indirect protection from a contagious infectious disease that happens when a population is immune either through vaccination or through previous infection.
“This means even people who aren’t vaccinated, or in whom the vaccine doesn’t trigger immunity, are protected because people around them who are immune can act as buffers between them and an infected person,” says Gavi.
“Once herd immunity has been established for a while, and the ability of the disease to spread is hindered, the disease can eventually be eliminated. This is how the world eradicated smallpox, for example,” states Gavi.
According to epidemiologist and lecturer Ruth Nduati, herd immunity refers to a reduced percentage of susceptible individuals in a given population. “Herd immunity means most people in the population are no longer susceptible to disease because they have immunity either from a vaccine or from exposure to the infection,” says Prof Nduati.
The academic says four out of five people that get infected with Covid-19 will not experience severe, symptomatic disease.
“Some countries like the UK and Sweden thought if they allow their population to acquire infection and only lock down the high-risk ones, they will acquire herd immunity. Unfortunately, the number of infections and deaths became unacceptably high so they backed away from this plan,” she said.
In an opinion written by Prof Daniel Altmann and published in the Lancet medical journal on protective immunity, the researcher says most of the data on infection cases has been obtained from patients hospitalised with Covid-19.
“In this group, around 90 per cent develop antibodies within the first two weeks of symptomatic infection and this appearance coincides with the disappearance of the virus, supporting an underlying relationship between these events,” Prof Altmann wrote in the April 27 article.
The medic says the current strategy focuses on infection prevention measures such as handwashing, social distancing and wearing masks, development of a vaccine and possible issuance of immunity certificates to those tested as a condition to return to work. “The essential pillars for a strategy to move out of lockdown seemingly rests on increased testing and contact tracing, possible return-to-work permits based on immune status, redesigned based on new medicines and, finally, vaccination,” he wrote.
Prof Altmann says scientists can determine how much herd immunity is sufficient to mitigate subsequent substantial outbreaks of Covid-19 using the Reproduction Number (R0) method used in epidemic theory. Ro is defined as the average number of secondary infections produced by a typical case of an infection in a population where everyone is susceptible. The R0 for Covid-19 is believed to be about 2·2.
“On the basis of this estimated R0, the herd immunity calculation suggests that at least 60 per cent of the population would need to have protective immunity, either from natural infection or vaccination,” he says.
According to UK medical research website Health Knowledge, usually not all contacts are, however, susceptible to infection.
For example, due to prior infection which has given some individuals life-long immunity, or as a result of previous immunisation, the resulting average number of secondary cases per infectious case will be lower than the basic reproduction number. Prof Altmann, however, insists there is need to consider patients who have not displayed symptoms in the process of declaring countries’ economies fit to reopen. He also advises caution on the release of recovered patients into open society as some are still susceptible to reinfection.
According to Dr Zablon Kerima, a Moi University lecturer and biochemist, the concept of herd immunity that has been tried out with some success in Sweden is not a foolproof one. “Herd immunity occurs when far large numbers of people in a population, including the most vulnerable, are infected with the virus, they get sick and some recover by a natural process thanks to their robust immune system, while others will be overwhelmed by the disease,” says Dr Kerima.
He insisted on vaccination as a safer route to herd immunity and cautioned against a rush to reopen the country for economic activity.