At the start of the pandemic, the term “herd immunity” made headlines, along with a polarized discussion about how to achieve it.
Some groups were wedded to the now discredited notion of letting a dangerous virus pass through the population to reach the critical level of population immunity needed to reduce transmission.
But more serious conversation has focused on the prospect of achieving herd immunity through vaccination.
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It’s the idea that vaccines – when available and used at sufficient levels – could crush the transmission of the virus. This would lead to the possible elimination or eradication of SARS-CoV-2, the virus that causes COVID.
The promise was that it would herald the return of life to normal.
It’s understandable why this notion garnered so much attention, as it promised a full return to a COVID-free world. But in reality, it’s probably always been a pipe dream.
Over time, herd immunity has become even less accessible.
Here’s 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 develop.
In fact, just as a bushfire goes out when it runs out of fuel to burn, an epidemic begins to wane when the virus runs out of people to infect.
The level of vaccination coverage needed in a population to allow you to achieve herd immunity is the “herd immunity threshold”.
It depends on two main parameters – the infectivity of the virus and the effectiveness of the vaccine.
In short, the more infectious the virus and the less effective the vaccine, the more people need to be vaccinated to achieve herd immunity.
More and more out of reach
As the pandemic progressed, herd immunity via vaccination became increasingly out of reach. In fact, based on what we know of the virus variants currently in circulation, herd immunity via vaccination is mathematically impossible today.
In early 2020, we were grappling with the original strain of SARS-CoV-2, which was far less infectious than current circulating variants.
The original strain had an estimated R0 (basic reproduction number) of two to three. That is to say, a person infected with the virus would transmit it on average to two or three other people.
If we assume that we are working with a vaccine with 80% efficacy, this gives an estimate of the herd immunity threshold of 60-80%. In other words, 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 decline. Mathematically at least, it was not out of reach.
However, as we know, circumstances have changed dramatically during the pandemic, with the original SARS-CoV-2 virus being replaced by much more infectious variants.
Although estimates of variant infectivity are subject to some uncertainty, it is reasonable to assume that Delta has a reproduction number of around five and that Omicron could be around around 20, which places it among the most infectious diseases known.
Based on these numbers for Delta and Omicron, herd immunity threshold estimates range up 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 increasingly mathematically impossible as the pandemic has progressed.
That’s not all.
During the pandemic, we have learned more about the performance of vaccines in the real world and the nature of our immune response.
Vaccines do not block all transmission
Herd immunity via vaccination and the calculations above assume that vaccines stop transmission 100% of the time.
Although vaccines reduce transmission significantly, they do not completely prevent it. If we take this into account in our calculations, the challenge of obtaining herd immunity becomes more difficult again.
Immunity declines over time
Achieving herd immunity also assumes that immunity to COVID is maintained over the long term. But we now know that immunity wanes after vaccination and after natural infection.
So if immunity is not maintained, even if herd immunity was theoretically possible, it would only be transitory. Its preservation would only require a major effort, requiring the regular delivery of reminders to the entire population.
New viral variants
Then we saw the emergence of new variants capable of evading the immune response. Any change in the immunogenicity of new variants moves the targets away, further compromising our ability to achieve herd immunity.
So why do we bother to vaccinate?
Although achieving herd immunity through vaccination is no longer a realistic proposition, it needs to be put into perspective.
Vaccines go hand in hand with other measures Herd immunity is best viewed as a gradient rather than a binary concept. In other words, even if we do not reach the herd immunity threshold, the higher the proportion of the population vaccinated, the more difficult it becomes for the virus to spread.
Therefore, vaccination can combine with other behavioral and environmental measures (such as physical distancing, wearing masks and improving ventilation), to have a substantial impact on the ability of the virus to move. in the population.
Vaccines protect people
Despite the allure 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 the disease on the population.
In this respect, despite the decrease in protection against infection, vaccines seem to offer longer-lasting protection against serious diseases.
So getting vaccinated remains as important today as it ever has been. Right now, at the start of winter and with few COVID restrictions, it has never been more important to make sure you are fully vaccinated.
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