According to the Office for National Statistics Covid Infection Survey, 94% of adults have antibodies against Sars-CoV-2.
So why is the number of cases rising and why does Professor Sir Andrew Pollard, a vaccine star, claim that herd immunity for Covid-19 has become mythical?
The average number of people who have been infected with SarsCoV-2 is called the reproduction number R. This number is R0 (the base reproduction number) if everyone was infected, such as when an epidemic started. R0 was three for vanilla Sars-CoV-2; it was seven for the Delta variant.
Imagine that six of the seven people infected would be immune. The virus would then only spread to one person, and the epidemic would cease growing. In this scenario R would effectively be one. In theory, if 1 1/R0 is immune, then we attain herd immunity. Sars-CoV-2's 6/7 = 86%.
What's the problem? The first problem is that the formula doesn't accurately reflect real life. Immunity isn't evenly distributed and people don't mix equally. The proportion of people with antibodies, which includes children, is less than 94%. Third, the formula must sterilise immunity to stop infection of potential hosts. Vaccinations reduce the risk of severe illness, transmission, and subsequent infection.
Sars-CoV-2 is different from measles which has a high R0 of around 16, but full vaccination or survived infections will likely confer lifelong immunity. Measles can spread even when people are not immune. This is evident when measles-infected individuals close to others, as was the case when young people were exposed to the MMR virus in the early 2000s and began gathering at music festivals.
Sars-CoV-2 is now endemic. This means that there will be recurrent outbreaks in communities with low immunity. All of us will be at risk. This is a hard message for anyone who is anxious about Covid-19. Although herd immunity is not a realistic goal, every step toward it helps.
