Why We Can’t Stake Our Hopes on ‘Natural Herd Immunity’

• Allowing viral spread is not an effective strategy to achieve herd immunity.
• Low infection rates make herd immunity unattainable without a vaccine.
• Natural immunity against COVID-19 may be short lived, making herd immunity impossible without a vaccine.

When a sufficient proportion of the population is vaccinated against an infectious disease, the remaining part of the population is protected. This is because it becomes extremely difficult for a disease to spread and circulate within a population that is mostly protected – this phenomenon is known as herd immunity.

At the start of the pandemic, there was much discussion around the topic of herd immunity, but not in the context of vaccination – even now, a vaccine remains some way off. Instead, some governments contemplated encouraging ‘natural herd immunity’ by simply allowing the virus to spread through the population – a ”take it on the chin” approach. Allow enough people to be infected and become immune, and the virus will go away. This way of thinking is ultimately flawed, and while few governments actually followed this plan, the idea that ”natural herd immunity” will save us has persisted. Unfortunately, this is highly unlikely to happen.

By now, many will be familiar with why the ”take it on the chin” approach is dangerous. A spike of cases that occur in a short period of time risks overwhelming health services and causes more deaths than the same number of cases occurring over a longer time period. Yet even with social distancing and other precautions, the virus will continue to spread, so why won’t we eventually reach herd immunity?

To illustrate the problem, we can look to Sweden, a country that did actively pursue herd immunity. Swedish authorities banned large gatherings and encouraged individual precautions, but stopped short of closing bars and restaurants. In April, a Swedish Ambassador boasted that the country could achieve herd immunity by the end of May. Yet by July, only an estimated 6% of the population had antibodies against the virus, while suffering significantly higher death rates and economic damage than its Nordic neighbours. Other countries have similarly low levels of immunity: in Spain, a country that was hit badly by the outbreak, only around 5% of the population have antibodies. Estimates of the level of immunity required to achieve herd immunity generally range from 50% to 80% of the population.

Why are these numbers so low? One reason is that the virus is spreading relatively slowly. Even in the absence of enforced social distancing, most people don’t want to expose themselves to the disease, and will take precautions to protect themselves. South Korea currently experiences around 50 new cases a day. At that rate, it would take roughly 3 years for 0.1% of the population to become infected, and fewer still would actually be immune.

This brings us to the next major problem: herd immunity requires a large proportion of the population to have robust immunity at any given time. There is still much we don’t yet know about COVID-19 immunity, but one thing that studies continue to suggest is that antibodies against the virus are short lived, especially in those with mild symptoms who make up the majority of cases. This does not necessarily mean that immunity is also short lived – antibodies are not the only way the immune system can defend against a virus, and Covid hasn’t been around for long enough to get conclusive proof of reinfection. However, if Covid is like other coronaviruses, we probably will see some reinfections within 12 months. If even mild reinfections can occur, natural herd immunity will not be possible.

In summary, natural herd immunity against a viral disease like COVID-19 is not attainable. When we have reached herd immunity against a disease in the past, such as during eradication efforts, we have done so through vaccination. We must therefore hope for an effective vaccine against Covid, but even this may not be enough. To reach herd immunity, vaccine uptake would have to be high, and recent surveys in the UK and US are not encouraging in this regard. One thing is next to certain, however: this virus will not go away by itself.