At the onset of the pandemic, scientists hypothesized that about 60 to 70 percent of people may need to be immunized against the virus to reach the “herd immunity” threshold.
This is the point where so many people are immune to the virus that it can no longer spread in the population and declines.
Obviously, this has not happened, as a third wave of coronavirus infection is still raging in the UK. Why?
What is collective immunity?
If a large number of people become immune to a disease, either by developing natural resistance or by vaccination, it is less likely to spread.
The virus needs each case to infect at least one other in order to maintain itself. If this does not happen – if the reproduction or the R number drops below 1 – it goes into decline.
Different diseases and variants have different R numbers, and the number will vary depending on other factors, such as control measures that reduce contact between potential spreaders.
How many people are immune?
According to the Office for National Statistics, it is estimated that around nine in ten people in the UK have antibodies to the SARS-CoV-2 virus, either through infection or vaccination.
This in part reflects the extent of the vaccine’s rollout across the UK. At the time of writing, 88% of the adult population had received one dose of the vaccine, while 69% had received both doses.
Not as good as it looks
While all the vaccines used in Britain have been shown to be effective in reducing deaths, hospitalizations, serious illnesses and infections, none of them are 100% effective.
Getting vaccinated does not entirely prevent transmission of the virus, especially in the form of the now dominant Delta variant.
And there is good evidence that a positive test for antibodies does not mean that you are completely protected against reinfection.
We must also remember that the vaccination figures only cover the adult population, but children play a role in the spread of the disease.
Even if we were to vaccinate all adults in the country – and we’re a long way from that goal – it would only represent about 70% of the total population.
Taking a range of factors like this into account, modelers at the University of Warwick said last week that up to a third of the UK population could still be susceptible to the Delta variant.
Blues of the delta
This suggests that about two-thirds of us have immunity – bringing us closer to the herd immunity threshold of 60-70% estimated by some scientists at the start of the pandemic.
A big change since then is the rise of the Delta variant, which is much more transmissible than the early forms of the virus and makes those early estimates overly optimistic.
Modelers believe that the R-number of the Delta variant could reach 6 to 8, which means that a single case can infect six to eight other people.
This implies that we would need to immunize 83% to 88% of the entire population, including children, to reach the collective immunity threshold and stop the spread of the disease.
And that would only work if the vaccines were 100% effective in preventing subsequent infections – which we know they are not.
Dr Adam Kucharski from the London School of Hygiene & Tropical Medicine told us: ‘Obviously there have been a lot of natural infections in the UK too, so this will help make up the difference, but it illustrates how point the threshold is raised to obtain R below 1. ”
Vaccines still work
Scientists say it’s important to understand that vaccination still helps slow the spread of Covid.
Dr Louise Dyson of the University of Warwick told FactCheck: “The herd immunity threshold is the point at which there is enough immunity in the population that we can lift all measures and cases continue to rise. lower – so R is less than one.
“This threshold is very difficult to know with precision, and I would like to urge caution in citing a given percentage of the population as the threshold for collective immunity.
“However, even without reaching the collective immunity threshold, the level of immunity in the population still has an effect. The more immunity there is, the fewer infections we will see at the same level of measurements. “
Professor Graham Medley of the London School of Hygiene & Tropical Medicine, chairman of the SPI-M group of expert advisers to government, said: ‘This is not an on / off process – collective immunity comes into play at as immunity increases.
“People in England are in contact with roughly half of the people we were before the pandemic.
“We estimate that at full contact rates and without immunity, the R for Delta would be around 7. At half the contact rates, we would expect R to be around 3.5.
“But we’re actually seeing R around 1.4, and the difference is immunity – so immunity reduces the R-value by around 60%. Without immunity, we would have a huge epidemic. “
Will it ever end?
Many scientists were pessimistic from the start about the chances of us reaching the collective immunity threshold and eradicating Covid-19, even with the rapid development of effective vaccines.
Professor Paul Hunter of the University of East Anglia said: “The grandchildren of our grandchildren will get the infection. Looking at other coronaviruses, we can expect repeat infections every four to six years, probably more frequently this decade.
“But don’t despair. We know that second infections and infections after vaccination are usually less severe than first infections, so the vast majority of these infections will be asymptomatic or a mild cold. “
Professor Willem van Schaik of the University of Birmingham told FactCheck: “The focus on ‘collective immunity’ has been somewhat confused since the start of the pandemic because the underlying assumption behind the claims of Herd immunity was that those who had antibodies to SARS-CoV-2, either through vaccination or a previous infection, would be fully protected against reinfection and would no longer transmit the virus.
“This is true for a number of infectious diseases (measles being probably the most important example), but SARS-CoV-2 is a different type of virus that can still infect people who have antibodies. Importantly, during infection, symptoms will usually be mild in these people, but they will still be able to transmit the virus.
“It is becoming increasingly clear that we will have to deal with SARS-CoV-2 for many years to come, and possibly decades to come. It therefore remains of crucial importance for as many people as possible to be vaccinated against COVID-19 as it provides protection against serious illnesses. “