COVID-19 may never go away, but practical herd immunity is within reach

When people say that we will not achieve “collective immunity”To COVID-19, they generally refer to an ideal of“ total ”population immunity: when so many people are immune that, most of the time, there is no community transmission.

With full herd immunity, most people will never be exposed to the virus. Even those who are not vaccinated are protected, because it is so unlikely that an introduction will reach them: it will spit, because so many others are immune – as is currently the case with diseases like polio and mumps.

The fraction of the population that needs to be immunized for the population to have “full” herd immunity depends on the transmissibility of the virus in the population and the control measures in place.

We are unlikely to achieve full herd immunity for COVID-19.

On the one hand, it appears that immunity to COVID-19 acquired either by vaccination or infection decreases over time. In addition, SARS-CoV-2 continue to evolve. Over time, variants that can infect immune people (even if this only results in mild disease) will have a selective advantage, just as so far selection has mainly favored variants with a higher transmission potential. .

In addition, our population is made up of different communities, workplaces and environments. In some of them, the risk of transmission may be high enough and / or the immunity low enough to allow larger epidemics to occur, even though in the general population we have a vaccination rate high and low transmission.

To finish, SARS-CoV-2 can infect other animals. This means that other animal populations can act as a “reservoir”, allowing the virus to be reintroduced into the human population.

Practical collective immunity

Nevertheless, we are likely to achieve a practical type of herd immunity through vaccination. In herd immunity practice, we can reopen to near normal activity levels without the need for widespread distancing or lockouts. It would be a profound change from the situation we have been in for the past 18 months.

Practical collective immunity doesn’t mean we never see COVID-19. It will likely be with us, just at levels low enough that we don’t need to put in place widespread distancing measures to protect the healthcare system.

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The level of immunity (by vaccination or infection) that we need for practical herd immunity is uncertain, but it can be quite high. The original strain of SARS-CoV-2 was highly transmissible and transmission is believed to be even higher for some variants of concern.

The level of immunity we need will also depend on how much control we are willing to maintain indefinitely. Continuous masking, contact tracing, symptomatic and asymptomatic testing, and outbreak control measures will mean we will need less immunity than without these measures.

Some estimates suggest that we might need two thirds of the population be protected either by successful vaccination or by natural infection. If 90% of the population is eligible for vaccination and vaccines are 85 percent effective against infectionwe can get those two-thirds with about 90 percent of the eligible population being vaccinated or naturally infected.

The UK has already exceeded these rates in certain age groups. Higher rates are even better, as there is still uncertainty about the level of transmissibility and effectiveness of the vaccine against infection (although research shows they are very good. against serious diseases). We don’t want to find out that we don’t have enough immunity from the vaccination and that we have another serious wave of infection.

Emerging variants

A higher vaccination rate means there will be fewer infections before achieving practical herd immunity. The remaining unvaccinated people will be safer, indirectly protected by the immunity of those around them. Epidemics will be smaller and rarer, and there will be fewer opportunities for vaccine variants to escape onset and spread.

That said, variants of SARS-CoV-2 will continue to emerge, and selection will favor variants that are beyond our immunity. Vaccine developers will continue to broaden the spectrum of available vaccines, and the boosters will hopefully allow us to maintain a practical long-term herd immunity.

It is possible that a variant immune evasion will emerge that is severe enough and transmissible enough to cause a new pandemic for which we do not even have practical herd immunity. But unless we may not be free from COVID-19, we can be sure that in the not-so-distant future it will be manageable when we return to near-normal life.