Why herd immunity no longer being sold as the path out of the pandemic

Early in the pandemic, the term “herd immunity” made headlines, along with a polarized debate about how to achieve it.

Some groups were attached to the now-discredited idea of ​​ripping a dangerous virus through the population to achieve the critical level of population immunity needed to reduce transmission.

But a more serious conversation turned to the prospect of getting herd immunity through vaccination.

This is the idea that vaccines – if available and used in sufficient quantities – can suppress the transmission of viruses. This would lead to the possible elimination or eradication of SARS-CoV-2, the virus that causes COVID.

The promise was that this would herald the return of life.

It’s understandable why this notion received so much attention, as it promised a full return to a world without COVID. But in reality it was probably always a utopia.

As time went on, herd immunity became even less attainable.

This is why we don’t talk about it anymore, even with the high vaccination rates we see today.

What is Herd Immunity?

If enough people in the community develop immunity to an infectious agent such as a virus, an epidemic cannot grow.

In fact, just as a wildfire goes out when it runs out of fuel to burn, an epidemic begins to subside when the virus runs out of susceptible people to infect.

The level of vaccine coverage needed in a population to get you across the border to achieve herd immunity is the “herd immunity threshold.”

This depends on two important parameters: the infectivity of the virus and the effectiveness of the vaccine.

In short, the more contagious the virus and the less effective the vaccine, the more people you need to vaccinate to achieve herd immunity.

More and more out of reach

As the pandemic progressed, herd immunity via vaccination became more and more out of reach. Based on what we know about the currently circulating viral variants, herd immunity via vaccination is mathematically impossible.

In early 2020, we struggled with the original strain of SARS-CoV-2, which was much less contagious than the current circulating variants.

The original strain had an estimated R0 (base reproduction number) of two to three. That is, someone infected with the virus would spread it to an average of two to three others.

Assuming we were working with a vaccine with an effectiveness of 80%, this gives an estimate of the herd immunity threshold of 60-80%. That is, when the original strain of the virus was circulating, we would have had to vaccinate 60-80% of the entire population to see the epidemic diminish. Mathematically at least this was not out of reach.

However, as we know, conditions have changed dramatically over the course of the pandemic, with the original SARS-CoV-2 virus being replaced by many more contagious variants.

Although estimates of the infectivity of the variants are subject to some uncertainty, it is reasonable to assume that Delta has a reproduction number of about five and Omicron may be in the margin of about 20, making it one of the most well-known infectious diseases.

Based on these figures for Delta and Omicron, the estimates of the herd immunity threshold range to 100-118%.

Since you can’t vaccinate more than 100% of the population, you can see how relying on vaccination to achieve herd immunity has become mathematically impossible as the pandemic has progressed.

That’s not all

Over the course of the pandemic, we’ve learned more about how the vaccines have performed in the real world and the nature of our immune response.

Vaccines do not block all transmission

Herd immunity via vaccination and the above calculations assume that vaccines stop transmission 100% of the time.

While vaccines significantly reduce transmission, they do not completely prevent it. If we include this in our calculations, the challenge of achieving herd immunity becomes more difficult again.

Immunity declines over time

Achieving herd immunity also presupposes long-term preservation of immunity against COVID. But we now know that immunity declines after vaccination and after natural infection.

So if immunity is not maintained, even if herd immunity were theoretically possible, it would only be transient.

Preserving it would only require significant effort, requiring regular delivery of boosters to the entire population.

New viral variants

Since then, we’ve seen new variants emerge with the ability to evade the immune response. Any change in the immunogenicity of new variants moves the goalposts further away, compromising our ability to achieve herd immunity to an even greater degree.

So why do we bother to vaccinate?

While achieving herd immunity through vaccination is no longer realistic, it should be put into perspective.

Vaccines go hand in hand with other measures

It is better to think of herd immunity as a gradient rather than a binary concept. That is, even if we don’t meet the herd immunity threshold, the greater the proportion of the population that is vaccinated, the harder it becomes for the virus to spread.

Therefore, vaccination can be combined with other behavioral and environmental measures (such as physical distancing, wearing masks and improving ventilation) to significantly affect the virus’ ability to move through the population.

Vaccines protect individuals

Despite the appeal of herd immunity, the primary goal of COVID vaccination has always been to protect individuals from serious illness and death, and thus the impact of illness on the population.

In this regard, despite decreasing protection against infection, vaccines appear to provide more durable protection against serious diseases.

So getting vaccinated now remains as important as it always has been. Right now, with the onset of winter and with few COVID restrictions, it has never been more important to make sure you are fully vaccinated.The conversation

Hassan Vally, associate professor, epidemiology, Deakin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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