Herd immunity: are we there yet?

This week’s easing of lockdown has been accompanied by encouraging figures on coronavirus immunity. 

Over half of the UK population now has Covid-19 antibodies, according to the latest Office for National Statistics (ONS) findings. In England, the figure sits at 54.9 per cent. Immunity may be even higher. The ONS report said: “It’s possible that antibody levels in some people are now too low to be detected by our tests but still high enough to grant some level of protection.” The results come from the ONS’s Covid-19 Infection Survey (CIS) – sponsored by the University of Oxford – in which participants are given nose and throat swabs weekly or monthly so that changes in antibody levels can be tracked.

Both vaccination and natural infection are responsible for the high levels of antibodies circulating within the British population. But what do these antibody figures mean for herd immunity? 

Herd immunity is achieved when enough of a population has protection against an infection that it stops being able to spread – meaning even those who don’t themselves have immunity are indirectly protected. 

The concept is straightforward, but the trickier part is determining what percentage of a population needs antibodies in order to stop the spread – the point at which herd immunity kicks in. Thresholds vary for different infectious diseases, but most estimates for Covid-19 are somewhere between 65 and 75 per cent. 

We have already reached it, according to optimistic modelling published by a team of scientists from University College London (UCL). The team, headed by Professor Karl Friston, argue that Britain passed the threshold for Covid-19 herd immunity on Monday 12 April. Their results put antibody levels significantly higher than the ONS’s 50 per cent mark – according to the UCL modelling, nearly three quarters of the population already have protection from the virus. 

Prof Friston told the Telegraph that the herd immunity estimates “surprised” him. But, he added, “they are unremarkable when one considers that over 50 per cent of adults have been vaccinated, around 42 per cent of people have now been exposed to the virus and about 10 per cent have pre-existing immunity [due to previous infection from other coronaviruses].”

It’s possible the UCL model is too optimistic. Scientists at Imperial College London are more cautious. According to their figures, by the end of March, only 34 per cent of the population had developed immunity to the virus. 

While reports of high levels of antibodies come as welcome news, many ambiguities about herd immunity remain. In addition to the divided opinions on the level of immunity present in the population, the levels required to achieve herd immunity are neither clear-cut, nor static.

They will be lower, for example, in summer when more people are mixing outdoors. The same level of antibodies which achieves herd immunity within a population over a warm July may not be enough to stop people catching Covid in the grips of winter. 

Crucially, the herd immunity threshold is also complicated by the emergence of new variants. The so-called Kent variant acquired mutations which enabled it to spread more easily. With a highly transmissible Covid strain, a greater level of vaccination and/or prior infection is required in order to reach herd immunity. 

We have also seen that the key variants of concern reduce the vaccine’s ability to stop Covid transmission. Research by AstraZeneca on the South African variant shows that vaccination is still a highly effective shield from serious illness. What the vaccine fails to do so effectively with the South African variant is block infection all together. This reduced ability to halt transmission of the virus will make it harder to achieve herd immunity. 

It’s also important to remember that herd immunity is not a fixed state, because immunity from the virus wanes over time. Even after reaching the threshold, a population could dip in and out of herd immunity depending on the state of play with booster shots and vaccine tweaks. 

Evidence of herd immunity around the globe is mixed. Following a devastating first wave of Covid last summer, the Brazilian city of Manaus was branded the first city in the world to reach herd immunity. In October, tests on donated blood suggested 76 per cent of the population had developed antibodies. 

However, in January, a second wave swept across Brazil, and hospitals in the Amazonas capital were once again overwhelmed. Somewhere between 25 and 61 per cent of those infected in Manaus had already had one bout of Covid – but prior infection failed to protect them from the troublesome Brazilian variant.

Yet Israel provides a more hopeful picture. Roughly 68 per cent of Israelis are estimated to have Covid-19 antibodies, thanks to prior infection combined with the successful vaccination of over half of the country’s population. Lockdown restrictions have virtually all been lifted, and many health experts believe herd immunity has been achieved. 

Prof Eyal Leshem, a director at Israel’s largest hospital, the Sheba Medical Center, has described herd immunity as the “only explanation” for the consistent fall in cases even as more restrictions were lifted. “There is a continuous decline”, he added, despite everyday life “returning to near normalcy.”

If the situation in Israel is anything to go by, then the UK is well on its way to achieving herd immunity.