SARS-CoV-2 and herd immunity – will the virus always be one step ahead ?

‘Herd immunity’ and vaccine strategy

The concept of ‘herd immunity’ is massively important in terms of disease transmission and overall efficacy of vaccine strategies. It relies on a certain proportion of a population having immunity against a disease, thus protecting those who are unable to receive vaccines. If vaccinated individuals stop the spread of infection, the most vulnerable will not be at risk, producing a collective immunity against the vaccinated pathogen.

The proportion of a population needing to be vaccinated to achieve herd immunity and mount antibodies against the coronavirus varies between diseases. The number depends on two major factors; the transmission rate of the virus and the efficacy of the vaccine. A recent paper in The Lancet estimates that, with a desirable vaccine efficacy of roughly 80%, we would need to vaccinate between 75–90% of the population to achieve herd immunity [1]. But COVID-19 is a complex disease, that is what we know for sure one year after the emergence of the pandemic. The duration of protection against the virus remains uncertain for the new COVID-19 vaccines. Preliminary evidence suggests decreasing antibody levels also in those individuals who have recovered from SARS-CoV-2 infection. However, we need to accept that antibodies are only one part of the “battle” against the virus, and cellular immunity seems also to play a role to prevent from reinfection

This figure poses a significant logistical challenge, due to distribution issues and variation in public trust in vaccinations. A survey by The University of Oxford determined that in the UK only 72% of the population would be willing to be vaccinated [2], with figures as low as 40% in France [3]. But this may change with the uprising of new more aggressive virus variants.

Viral mutation – variants of concern

As herd immunity relies on a certain level of vaccine efficacy, the recent mutations that have been detected in South Africa, the UK and Brazil [4] represent a threat to ever achieving full population protection. While mutations occur randomly, those which allow increased survival are more likely to persist, meaning that more resistant strains eventually emerge.

Current variants of concern are thought to have arisen by mutations allowing them to evade the immune response due to natural infection. This can be seen through reduced efficacy of antibodies produced by previously infected patients in combatting new strains [5]. As more individuals become vaccinated, it is quite probable that similar immune evasion strategies by the virus will arise against vaccine-induced immunity. This poses a threat, with new strains potentially reducing vaccine efficacy and preventing herd immunity from being reached.

Vaccine strategies – can they cope?

The concept of viral mutation is not new, and previous strategies seen with influenza vaccines have helped to control emerging strains which have the potential to cause epidemics. Seasonal flu vaccines contain a cocktail of variants which are predicted to be dominant in a given year. This is a strategy that could potentially be employed with coronavirus, provided that accurate predictions of future dominant variants could be made.

Seasonal vaccinations are effective in providing protection for vulnerable groups but cannot possibly protect entire populations. This is an issue in coronavirus spread, as the infection rate is comparatively higher than that of influenza [6] so, while seasonal vaccines could protect the most vulnerable, they would not be able to contain spread entirely. It is also important to consider the fact that emerging variant predictions are not always accurate. Despite influenza having a far lower mortality rate than SARS-CoV-2 infection [7], there are still quite a lot seasonal influenza deaths every year.

As we are currently seeing in vaccine rollouts across the world, inoculating an entire population is logistically extremely challenging. Although the contents of the vaccine can be modified to account for new strain emergence, the time taken to re-inoculate the entire population would be significant.

Long-term implications and the need for therapeutics

While vaccines are likely to provide population-wide protection against current strains of coronavirus, they may not be a permanent solution. The emergence of new variants and the logistical challenges associated with mass vaccination mean that therapeutics are still essential. It is also of utmost importance to develop effective therapeutics to capture and neutralize the coronavirus and its mutants. It is important that we combine spread prevention with a continued drive to effectively treat those infected, if we are to beat the virus once and for all.