What is herd immunity?

Herd immunity to a transmissible infectious agent is achieved when enough people in a population become immune to infection (either naturally or through vaccination) that transmission rates decline. Not every person in the population has to be immune for this to happen, but when the risk of those that aren’t immune coming into contact with someone transmitting the infection become so low that the disease is not able to spread/maintain transmission rate, then we have reached herd immunity.

Each infection differs in contagiousness and so has a different percentage of the population that needs to be immune to achieve herd immunity.

What about COVID-19?

Scientists estimated near the start of the outbreak in 2020 that 60-70% of the population might need to be immune to get herd immunity. However, it is becoming increasingly clear that this pandemic is not behaving as simply as it first seemed. Transmission rates remain high even though up to 95% of the UK population are thought to carry antibodies to COVID-19 and should be at least partially immune.

Why isn’t it happening?

• The vaccine doesn’t completely stop transmission

There are a number of reasons why herd immunity against COVID-19 is appearing to be an elusive target. Firstly, it is important to make clear that whilst the vaccine does reduce the risk of becoming seriously ill from the virus, it only partially blocks transmission. A new study suggests that a person who is fully vaccinated with the Pfizer vaccine and becomes infected with the delta variant is 65% less likely to pass it on compared to an infected non-vaccinated person, and a fully vaccinated person with the AstraZeneca vaccine is 36% less likely to pass it on. These are overall very positive results, especially because the delta variant of COVID-19 is more contagious than the alpha variant against which the vaccine was designed, yet they show that transmission can still occur even from a fully vaccinated person. Most people in the UK owe their immunity to one of the vaccines mentioned in the research article, so herd immunity will be difficult to achieve because our current vaccines don’t guarantee that spreading will be stopped even amongst vaccinated people.

Interestingly, according to the study mentioned above, after about 3 months from the 2^nd vaccine, the transmission doesn’t seem to be stopped at all and if you are infected you become just as likely to pass it on as someone who isn’t vaccinated. Still, if you are vaccinated you are much less likely to catch it and become ill so some benefits remain, and of course, there is a booster vaccine program after 6 months from the 2^nd vaccine which are intended to restore the block to transmission.

Read the full study here – https://bit.ly/3Be5FIu

• Variants

One of the main things stopping the eradication of COVID-19 are new viral variants. A new variant such as delta is capable of causing new waves of infection, especially when vaccine rollout is not yet complete. These new variants may be more transmissible and can be slightly more vaccine-resistant. Therefore, it is key for the majority of the population to become immune before these new variants have time to adapt and emerge.

• Vaccine rollout

Vaccine rollout has not been even among age groups nor between countries. At time of writing in the UK (October 2021), about 79% of people over 12 are double vaccinated. However, vaccinations for 12–15-year-olds didn’t start until September 2021 and those under 12 are not yet being offered a vaccine. This means that whilst the older, more vulnerable people have been vaccinated for some time, the virus has now begun spreading among unvaccinated children (who could then pass it on to their older relatives/people they are in close contact with). In other words, the virus still has a large unprotected population group to infect who have low immunity and so it can continue to spread. Not only this, but although the UK is ahead in terms of vaccine rollout, other countries are not so immune and international travel will continue to be a route by which COVID-19 and its variants are able to spread worldwide.

Vaccine hesitancy has been another big issue in preventing herd immunity. The more people that don’t get vaccinated, the less likely it is that herd immunity will be achieved as it relies on the majority of the population being immune.

• Social behaviour

In the UK, most restrictions are now lifted and mask-wearing and social distancing are no longer legal requirements. Inevitably, people mixing means the virus is now able to spread more easily amongst those that aren’t yet immune and that otherwise wouldn’t have come into contact had restrictions still been in place. In an ideal world, restrictions would remain until herd immunity is achieved, however, this seems unrealistic.

So what does this mean for the future?

Eradication of COVID-19 through herd immunity is not as close as we’d once hoped; it seems we must adapt to a new normal and accept that this virus may be with us for some time as immunity naturally waxes and wanes – much like it does to the common cold and flu. That being said, the success of the vaccination program in the UK would suggest that the worst of the pandemic in terms of severe illness and death is mostly behind us (the prospect of newer, more deadly variants optimistically put aside for now) and as immunity continues to increase with booster vaccines and further natural exposure to COVID-19 infection, it will no longer be such a deadly virus.
Some scientists predict that COVID-19 may become something like the flu with seasonal peaks causing low numbers of hospitalisations and death. Perhaps ‘herd immunity’ will become an annual cycle of vulnerability to infection (though not necessarily vulnerability to severe symptoms), with natural immunity backed up by annual booster vaccines.