Why herd immunity against Covid-19 is hard to achieve

As the coronavirus continues to evolve, it remains uncertain how selective pressures, including the vaccines, will shape its path.

The initial generation of vaccines appears less likely to prevent mild to moderate disease from many of the variants that have emerged.

According to studies, the vaccines may still allow people to become infected by and transmit the virus even if they do not develop symptoms.

While they minimise the risk of disease, the vaccines may be less effective at fully preventing infection and transmission. Also, the duration and efficacy of immunisation for those who have recovered from the virus are not yet fully understood.

We have seen evidence suggesting that people who have recovered may be reinfected by new variants of concern (VOC) because their immunity has faded, or because VOC make their immunity less effective, or both.

We must, therefore, begin to confront the dawning and daunting reality that the disease will most likely continue to smolder, like the flu, long after the acute, deadly pandemic phase has waned. And should even be planning permanent processes to conduct sustained testing and perform regular booster shots. Reducing the risk of disease and treating it as endemic isn’t by itself enough.

Another strategy could be herd immunity. Usually, when a sufficient number of people have recovered or been vaccinated, and fewer people are left to infect, outbreaks fail to take hold across a population and the disease retreats into the background.

Estimates vary, but it usually takes 60 per cent to 80 per cent of the population to be immune to disease, infection, and transmission, either naturally or via vaccination.

Yet, if indeed Covid is endemic and continues to circulate, herd immunity is a distant and perhaps unattainable goal.

There are reasons why minimising infection and transmission is especially challenging with the current vaccines and emerging variants.

First, it’s unclear whether the current vaccines prevent transmission. While they are generally effective at preventing symptomatic disease, there’s still a substantial amount of virus spread that would make it a lot harder to break transmission chains.

Secondly, distribution of the vaccine roll-outs has generally been even (mostly stratified by age, with priority given to older people, who are at the highest risk).

In Kenya, for instance, huge variations in the efficiency of roll-outs, coupled with geographical clustering of infections, would most likely make the path to herd immunity a lot less of a straight line.

Another hindrance is the emergence of new variants that aren’t only more transmissible, but which, according to studies, are also resistant to vaccines.

The existing data about other coronaviruses, and the preliminary evidence for SARS-CoV-2, also seems to suggest that infection-associated immunity wanes over time. Another challenge is that vaccines may change people’s behaviour.

As more people are vaccinated, they will increase their interactions, and that changes the herd-immunity equation, which relies in part on how many people are being exposed to the virus.

Under these circumstances, the continued pursuit of herd immunity would almost certainly delay progress toward a full recovery from the pandemic. If our recovery is tied to reaching certain herd immunity thresholds, we will likely remain shut down longer than if we vaccinated the vulnerable and then protected the rest of the population.

Alongside that, we should keep sufficient public health measures in place, even after the vulnerable are vaccinated. This will help contain the virus, minimise disease risk, and give all those who choose to be vaccinated a chance to do so.

Once we achieve these aims, public health measures can be more fully relaxed. By focusing on the vulnerable and a disease risk mitigation strategy, the government can likely shave months off what should be a gradual recovery timetable.