In May 2020, we and other scientists predicted that many regions of the world might never reach the herd immunity threshold for Covid-19, the point at which enough people are immune to infection that transmission begins to slow down.
Even as vaccines have become more accessible in wealthy nations, this is still true.
The herd immunity threshold was seen as a universal target to hit early in the Pandemic. The threshold has always been changeable: it depends on how transmissible the pathogen is, how much they mix, and how easily they are spread.
A large proportion of the population will need immunity to stop the spread of a transmissible virus if it is very transmissible or the population is very densely packed. Fewer people need to be immune to slow the virus down if it is less transmissible or if a population doesn't mix often. The herd immunity threshold would be different in each case.
The herd immunity threshold for the original Sars-CoV-2 virus strain was estimated to be 70%. Delta and Omicron are more recent and more transmissible, so it is possible that they have thresholds of up to 80%.
Even though Omicron's existing immune protection against infections and transmission is low, we wouldn't likely see Omicron transmission dying out even with the majority of the population vaccinations.
Changes such as wearing masks, working from home and avoiding large gatherings can slow the spread of the virus. The herd immunity threshold will be raised compared to a more cautious population as interventions are lifted. If people live in crowded housing, Covid may raise the herd immunity threshold locally to make up for the disproportionate toll it has already had on marginalised communities.
Scientists suggested last year that we were approaching herd immunity through a combination of vaccine and infections. The largest spikes in cases to date in many regions are in places where the population immunity from infections is high. The Delta and Omicron variants have recently caused a surge in their transmissibility and ability to partially evade immunity, making a lot of us vulnerable to infections again.
Despite the success of vaccines, we have yet to reach the herd immunity threshold. We have to not resign ourselves to endless eruptions.
The transition to Covid-19 endemicity is awkward. In a paper published in Science in January of 2021, the authors predicted that Covid-19 would eventually transition to a mild infection that would continue to circulate in the human population as people become exposed. An endemic virus is what it is.
Once Covid-19 is endemic, immunity against severe disease would last longer, causing infections and mild disease to shift into younger age groups yet to be exposed or immunised, and instances of severe disease.
We are not at that point yet. The authors warned that behavioural interventions were still necessary to slow the spread of long Covid, which is now estimated to affect 1.3 million people in the UK alone. As we transition to endemicity, Vaccination and periodic boosting are important for blunting the worst outcomes.
It is important to remember that the herd immunity threshold cannot be reached, but that the build up of immunity within a population can. Severe disease is reduced. People who are vaccined may become infectious for less time and have lower viral loads. Protection is multiplicative when people who have been vaccined come in contact with each other, as the chances of them becoming infections are reduced.
It is still possible that the spread of Covid-19 in undervaccinated regions could lead to the evolution of new variants that cause disease. Competition between the different strains of the virus may work in our favor, as Omicron has been shown to cause less severe disease than previous strains. If they are highly transmissible, milder versions still have the potential to inundate hospitals. Rapidly vaccinating and boosting the global population is one of the best ways to slow the emergence of new variant and the uncertainties and setbacks they bring.
We can prepare for Covid-19 by improving access to vaccines, high-quality masks and tests, and requiring them in public settings where risk is high. We need to invest in research and distribution of treatments to help people with long-term effects of Covid-19. We need to strengthen our public health infrastructure to prevent future Pandemics, establish regulations for safer work environments, and mitigate health disparities that have been caused by the Pandemic. We can manage Covid-19 with smart policy and collective action.
An assistant professor of biology at the university, Mordecai is also a researcher. Harris is a PhD candidate at the university.
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