But if we’re all vaccinated, we’ll be safe from the worst
In early 2020, during the first thrashes of the pandemic, we were all talking about herd immunity.
At that stage, many commentators were arguing we should let COVID-19 rip through populations so we could get enough people immune to the virus that it would stop spreading. As I argued at the time, this was a terrible idea that would overwhelm hospitals and gravely sicken and kill many people.
Now we have safe and effective vaccines, we can aim to reach herd immunity in a much safer way. It’s certainly possible we’ll be able to reach and maintain local herd immunity in certain regions, states and countries. However the pandemic ends, it will involve this immunity to some extent.
But it’s still very uncertain whether long-term, global herd immunity is achievable. It’s quite likely the coronavirus could continue to spread even in places with high proportions of their populations vaccinated. It will probably never be eliminated.
However, if we’re all vaccinated, we’ll be largely safe from the worst ravages of the infection even if it does break out.
What is herd immunity again? And what does it mean for us long-term?
There are a few different definitions of herd immunity. Nevertheless, they all deal with the “reproductive number” of a disease, known as the R number. This is the average number of people an infected person will pass a disease on to, at a certain point in time.
The R number depends on how infectious a disease is. Measles is often used as an example, because it’s one of the most infectious diseases. In a group of people among whom no one is immune to the disease, on average one person will pass measles on to around 15 others.
But as more people in the community become immune, either through vaccination or getting the disease and recovering, each infected person will pass on the infection to fewer and fewer others. Eventually, we reach a point at which the R number is below 1, and the disease starts to die out. The R number falling below 1 here is in a population where there are no social restrictions, so the disease starts to die out because of immunity and not because of measures like lockdowns. This is one definition of herd immunity.
However, another potential definition is that herd immunity is a state where enough people are immune in a population that a disease won’t spread at all. One of the more confusing parts of the pandemic is we scientists haven’t always used the same definition across the board.
For example, when we say “reached the herd immunity threshold”, we could be talking about a transient state where we’re likely to see another epidemic in the near future, or a situation where the vast majority of a population is immune and thus the disease won’t spread at all. Both are technically “herd immunity”, but they’re very different ideas.
How’s herd immunity calculated?
COVID-19 has an R number somewhere between 2 and 4 in groups of people where no one is immune. Using a simple mathematical formula, 50-75% of people need to be immune to COVID-19 for the R number to fall below 1 so it starts to die out, in a population with no social restrictions. Some researchers have done more complex versions of this calculation throughout the pandemic, but that’s the basic idea behind them all.
However, herd immunity is a moving target. For example, if everyone in your local population is taking great care to socially distance, COVID-19 won’t spread as much. Therefore, in practice, different cultures spread diseases to different extents, so the R number varies in both place and time.