Figures out today suggest that as many as 90 per cent of people across the UK have Covid-19 antibodies, either from infection or vaccination.
At the beginning of the pandemic, scientists speculated that around 60 to 70 per cent of people might need to be immune to the virus to hit the “herd immunity” threshold.
This is the point where so many people are immune to the virus that it can no longer spread through the population and goes into decline.
Clearly this hasn’t happened, as a third wave of coronavirus infection is still raging in the UK. Why?
What is herd immunity?
If large numbers of people become immune to a disease, either by developing natural resistance or through vaccination, it has fewer chances to spread.
The virus needs every case to infect at least one other to sustain itself. If that doesn’t happen – if the reproduction or R number falls below 1 – it goes into decline.
Different diseases and variants have different R numbers, and the number varies according to other factors, like control measures which reduce contact between potential spreaders.
How many people have immunity?
According to the Office for National Statistics, about nine out of ten people across the UK are estimated to have antibodies to the SARS-CoV-2 virus, either from infection or vaccination.
This partly reflects the scope of the vaccine rollout across the UK. At time of writing, 88 per cent of the adult population had been given one dose of vaccine, while 69 per cent had received both doses.
Not as good as it sounds
While all the vaccines in use in Britain have proven to be effective in reducing deaths, hospitalisation, serious illness and infection, none of them is 100 per cent effective.
Getting vaccinated doesn’t entirely prevent onward transmission of the virus, particularly in the form of the now-dominant Delta variant.
And there is good evidence that testing positive for antibodies doesn’t mean you are completely protected against reinfection.
We also need to remember that the vaccination figures only cover the adult population, but children play a role in spreading the disease.
Even if we vaccinated every single adult in the country – and we are some way off that goal – that would only add up to about 70 per cent of the entire population.
Taking into account a range of factors like this, modellers at the University of Warwick said last week that as much as one third of the UK population could still be susceptible to the Delta variant.
That suggests that around two thirds of us do have immunity – which brings us close to the 60-70 per cent herd immunity threshold some scientists estimated at the beginning of the pandemic.
One big change since then is the rise of the Delta variant, which is much more transmissible than early forms of the virus and makes those early estimates look too optimistic.
Modellers think the R number for the Delta variant could be as high as 6 to 8 – meaning a single case can infect six to eight other people.
This implies that we would need to vaccinate 83 to 88 per cent of the entire population, including children, to reach the herd immunity threshold and stop the illness from spreading.
And that would only work if the vaccines were 100 per cent effective at preventing onward infection – 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 contribute to making up the difference, but it illustrates just how high the threshold is to get R below 1.”
Vaccines are still working
Scientists say it’s important to understand that vaccination is still helping to slow the spread of Covid.
Dr Louise Dyson from the University of Warwick told FactCheck: “The herd immunity threshold is the point at which there is enough immunity in the population that we could lift all measures and still have cases falling – so R is less than one.
“That threshold is very difficult to know precisely, and I would urge caution about quoting a given percentage of the population as a herd immunity threshold.
“However, even without reaching the herd immunity threshold, the amount 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 measures.”
Professor Graham Medley from the London School of Hygiene & Tropical Medicine, the Chair of the SPI-M group of expert advisors to the government, said: “It’s not an on/off process – herd immunity kicks in as immunity increases.
“People in England are approximately making contact with about half the people that we were pre-pandemic.
“We estimate that at full contact rates and no immunity the R for Delta would be about 7. At half the contact rates, we would expect R to be about 3.5.
“But we are actually seeing R about 1.4, and the difference is immunity – so immunity is reducing the R value by about 60 per cent. Without immunity we would be having a huge epidemic.”
Will it ever end?
Many scientists have been pessimistic from the start about the chances of us reaching the herd immunity threshold and eradicating Covid-19, even with the swift development of effective vaccines.
Professor Paul Hunter from the University of East Anglia said: “Our grandchildren’s grandchildren will be getting the infection. Looking at the other coronaviruses, we can expect to get repeat infections every four to six years, probably more frequently this decade.
“But don’t despair. We know that second infections and infections after immunization are usually less severe than first infections, so the vast majority of these infections will be asymptomatic or mild common cold.”
Professor Willem van Schaik from Birmingham University told FactCheck: “The focus on ‘herd immunity’ has been somewhat confused from the start of the pandemic as the underlying assumption of herd immunity claims was that those that had antibodies to SARS-CoV-2, either through vaccination or prior infection, would be entirely protected from re-infection and would no longer transmit the virus.
“This is true for a number of infectious diseases (with measles probably being the most important example) but SARS-CoV-2 is a different type of virus that can still infect people that have antibodies. Importantly, upon infection symptoms will be generally mild in these individuals 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, and possibly even decades to come. It thus remains crucially important for as many people as possible to be vaccinated against COVID-19 as this provides protection against serious illness.”