We have been living with Covid now for nearly two years. Lockdowns, daily infection rates, isolation, mask-wearing, and social restrictions have become part of everyday life in Britain.

The production of vaccines offered hope of a way out of the pandemic; and the data suggests they have significantly reduced the severity of disease, hospital admissions, and deaths. However, people are still getting Covid, despite being vaccinated and boosted, and we are in the middle of yet another wave with the new Omicron variant. So, what is the future for Covid? Will we reach so-called herd immunity or require booster shots each winter? Are more variants and waves to come?

In short, herd immunity is when enough people in a population have immunity from disease, either through vaccination or infection, and have developed antibodies protecting them from getting the disease. The virus then has fewer hosts it can infect and replicate in, so transmission rates reduce dramatically.

Herd immunity thresholds vary for different viruses with some viruses; you generally get infected once, such as chickenpox, so it is more common in younger age groups. Some vaccine programmes can dramatically reduce rates of a disease in a population, e.g. measles.

The World Health Organisation (WHO) says herd immunity to measles is reached when 95 per cent of a population is vaccinated, as the other 5 per cent will be protected because vaccination is significantly reducing the spread of the virus. But if vaccination rates fall, then outbreaks will occur – as has been seen after Measles, Mumps, Rubella (MMR) vaccine rates fell. Herd immunity for Covid-19 was discussed at the start of the outbreak, but many scientists now feel we will not achieve herd immunity.

Covid-19 is a new infectious disease, and as we learned more about the virus, and cases of re-infection, estimates for the herd immunity threshold varied. But herd immunity is dependent on many factors such as how transmissible the virus is (we know Delta and Omicron were more transmissible than previous strains), the immunological state of the population (which changes as vaccination and infection rates increase immunity in the population) and how much a population interacts (lockdowns and work from home will have impacted this slowing the spread). So, it is easy to see how this is a constantly changing landscape.

For Covid, we know it is highly transmissible and when it first hit, there was little or no immunity in the population. Lockdowns to reduce spread were necessary to halt infections whilst scientists and doctors learnt about the virus.

So, nearly two years on, how likely are we to ever reach herd immunity?

The original Covid virus strain had a herd immunity threshold estimated at 60-75 per cent, however, the appearance of more transmissible variants, Delta and then Omicron, have pushed thresholds upwards to 80-90 per cent. We have also seen cases of re-infection and vaccinated people getting Omicron; so again, it is hard to see if herd immunity can ever be fully reached if there are significant rates of reinfection.

We know that as soon as people mix, cases start increasing, and if a new variant arises that can evade our immunity, then it will spread quickly. For these reasons, many scientists believe Covid has become so widespread there is little hope of eliminating it; we will have to learn to live with it.

Other human coronaviruses are endemic – continually circulating within populations – and cause multiple reinfections, but they produce sufficient immunity to protect against severe adult disease. One example of this is the common cold, which can be caused by a coronavirus. Children have more colds than adults as they build up their immunity, but even as adults, most of us can expect a couple of colds a year.

Although colds can make you feel pretty miserable, they are not generally the cause of severe illness. Many scientists feel this is how we will transmission with Covid, from waves of serious case numbers and severe disease to Covid being present in the community and outbreaks occurring, possibly more in winter months like flu, but that our immunity will be sufficient that we do not see huge cases of severe disease.

This may mean that the disease will shift to younger age groups, and children may need immunising to give them protection, as they have little or no immunity, as with elderly and more vulnerable groups.

However, we are not at that stage yet as we experience yet another wave of Covid. So how soon are we likely to live with Covid? Some scientists predict that Covid-19 will eventually reduce transmission into a milder infection that will circulate within human populations but at lower levels, due to acquired immunity.

In some ways, we are seeing this with Omicron, although more transmissible compared to the Delta variant, it seems to be associated with milder disease, and combined with a largely vaccinated population (plus immunity from infections) it is at this point causing fewer hospitalisations in the UK than the delta wave – but there are significant numbers in hospital placing a strain on the NHS. Scientists are s warning that it may not end with Omicron and other variants may arise, especially in unvaccinated populations where the virus can spread and mutate more easily.

This may be more transmissible than Omicron and cause more severe disease. Continual monitoring of variants worldwide will still be needed.

In the UK there are still significant numbers who have not received the booster Covid vaccine, so while immunity in the population is still being boosted, by vaccinations and prior infections, numbers will remain high and therefore risks for severe disease and hospitalisation will remain high. In addition, we do not fully know how long immunity lasts.
There is some evidence it wanes. So, some social interventions like mask-wearing, work from home etc. It may be necessary to slow the spread and help us through the waves without overwhelming the NHS.

Although herd immunity may not have been reached at the beginning of the pandemic, many scientists remain hopeful. As population immunity is increased, we will live with Covid, and it becomes a milder disease. Vaccination of vulnerable groups and boosters may be needed going forward, similar to the flu vaccine.

There is still the risk of new transmissible variants appearing, especially in under-vaccinated regions, that can get around our immunity but, in the meantime, we have to wait and hope this wave of Omicron proves to be milder in all age groups. Many of us will still get Covid despite vaccination and prior infections, but as our immunity to the virus increases, we should be protected from severe disease.

Rachel Kayani