Throughout Covid-19, hope has been pinned on reaching the herd immunity threshold. Defined as the proportion of people that need to become immune to a disease before the spread slows down, this was estimated to be around 60–70 per cent at the pandemic’s start.
Since the Delta variant emerged, however, that threshold is believed to have risen to over 85 per cent. And another factor is now complicating that statistic even further: does our immunity from vaccines decline over time? And if so, by how much? While these might sound like simple questions, they are difficult to answer precisely.
In July, the Israeli Ministry of Health compared the chances that vaccinated and unvaccinated people would develop symptoms and test positive for Covid-19. It claimed that the Pfizer vaccine’s efficacy dropped to 64 per cent as restrictions were lifted and the Delta variant spread, especially in regions with a younger and richer population.
Why did this happen? The reason is that vaccines don’t make people completely immune; they lower the chances of infection if they are exposed to the virus. Although the vaccines didn’t become less effective, the virus had far more opportunities to break through the protection they provided.
This “leakiness” means that the herd immunity threshold is tricky to understand. Since vaccines don’t offer complete protection, we might underestimate how many people would need to be vaccinated to slow down the spread of disease.
But there are also other challenges in establishing whether immunity wanes. It’s not straightforward to compare people who are vaccinated to people who are not, because unvaccinated people can also gain some immunity if they have been infected. As an outbreak grows and more people get infected, this proportion grows too, which means people who were vaccinated would gradually appear to have less of an advantage over people who were not. (It’s important to note that the immunity that people gain from being infected naturally is less than they would gain from vaccines, and their immunity may also wane over time.)
A third complication is that the efficacy of a vaccine is calculated as an average among the population, even though there are differences in how much protection they give each person. People who are immunocompromised, for example, gain less protection from the vaccines than those who aren’t. For this reason, if there are changes in who receives the vaccine, the average efficacy can change too.
In some countries, people who were older and more vulnerable were more likely to be vaccinated earlier. As the vaccine programme expanded, vulnerable people would make up a smaller proportion of all the people who were vaccinated, and the overall efficacy might appear to increase over time.
In practice, there are many factors surrounding vaccine efficacy that can occur at the same time and pull results in opposite directions. Therefore, the efficacy of vaccines might appear to increase or decrease over time, even if there was no actual change in how well they worked.
The good news is that there are ways to address these challenges. Scientists can account for the demographics that affect people’s risk of being infected. They can use antibody tests to find out whether people have been infected in the past, and use medical records to find out who might gain less protection from the vaccines. But the bad news is that the obstacles are still numerous, difficult to measure and hard to fully overcome.
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A recent study conducted by researchers at the University of Oxford, for example, addressed many of these factors – they accounted for differences in age, income, occupation, medical conditions and so on – and still found that the efficacy of the vaccines appeared to wane over time, with faster declines in the Pfizer/BioNTech vaccine than the Oxford/AstraZeneca one.
However, the factors above are difficult to measure precisely and account for correctly: only a proportion of people agreed to take part in the survey, and only a proportion of those had data on whether they had been infected in the past. There was also a lot of uncertainty around how to extrapolate the level of future waning.
But there are good reasons to believe that immunity lasts a long time, especially with regards to severe diseases. There is a natural decline over time in the level of antibodies that circulate in our bodies after any infection, but memory immune cells can become activated by an infection and replenish them when needed. Some of these immune cells mutate rapidly and adapt to different versions of the virus through a process called “affinity maturation”, which means they actually improve in their ability to tackle variants of Covid-19. Although this protection does not usually come soon enough to prevent early symptoms of disease, it reduces the chances that it will progress further and cause death.
Taken together, the precise amount that vaccine efficacy wanes is still unclear, but the chances that immunity declines is a reason to stay vigilant.
The ability of the Delta variant to spread more rapidly and cause higher mortality means that a higher proportion of people need to be protected to achieve herd immunity. The possibility of new variants or waning immunity increases this need even further. A reduction from 94 per cent to, say, 88 per cent efficacy might seem slight, but it would imply that the protection from disease has halved.
The world’s focus might currently be on establishing and reaching the herd immunity threshold. But to ensure true future resilience, the aim should be to exceed it.