Among the more reliable guides to the real prevalence of COVID-19 are the data collected on antibodies acquired following infection (measured as N-antibody levels, in contrast to S-antibody levels, which are acquired from both infection and vaccination).
Below is the latest graph from the UKHSA showing how antibody levels in blood donors in England have changed since autumn 2020.
From this and earlier UKHSA/PHE reports we can infer that the first wave infected around 5.7% of the population (though data from this period is mixed, with some showing up to 8.3%), the Alpha wave infected around 9.9% (perhaps a bit less if you use a higher first wave estimate – either way the first two waves together infected around 16%), the Delta wave infected around 8% and the Omicron wave has infected around 21.4% (so far). On these data, around 45% of the country have now been infected at least once at some point during the four Covid waves. Note this doesn’t allow for any waning of infection-acquired antibodies, which would mean these are lower-bound estimates.
Dr. Clare Craig dug into the figures to see how this broke down by age, producing this chart.
What lies behind the differences in the sizes of these waves – is it differences in the variants and susceptibility to them, seasonal changes, the effectiveness of restrictions, the impact of vaccination, or something else?
Most likely it’s a combination. The winter waves (Alpha and Omicron) are both larger than the non-winter waves, for example, suggesting a role for seasonality.
It’s hard to read much into the role of restrictions on these data. For both the first wave and Alpha wave, lockdowns were imposed at a time when new infections appeared to be peaking – though how far that was a result of voluntary behaviour change is a matter of debate. Importantly, there was no Alpha exit wave as restrictions were eased (this was true in other jurisdictions as well), indicating that it was not restrictions that were preventing the spread. The Delta wave occurred at a time when restrictions were being lifted, yet it remained relatively small. The Omicron wave had only light measures (mask mandate, vaccine passports and work-from-home guidance) implemented in response. It’s hard to see much of a pattern here.
What about vaccination? The Delta wave is smaller than the Alpha wave, which may be thought to reflect vaccine protection. However, UKHSA data from the period show that reported infection rates during the wave were often higher in the vaccinated than the unvaccinated. In the large Omicron wave, too, reported infection rates have been far higher in the vaccinated than the unvaccinated. This suggests the large size of the Omicron wave may be due to vaccination making people more susceptible to the variant. However, the Delta wave, where the vaccinated also reported higher infection rates, was relatively small, so the intrinsic transmissibility of Omicron (or increased susceptibility of the population to it) may have played a part, alongside a seasonal effect.
I think the big story here is the large size of the Omicron wave (nearly three times the size of the Delta wave, and still going) despite coming at a time when most of the country had been vaccinated, 99% of the adult population had Roche S antibodies (from vaccination or infection) and 24% had Roche N antibodies (from infection). The variant’s partial evasion of natural immunity may have played a part; however, note that the large increase in antibody prevalence indicates it was infecting a considerable number of people (over a fifth of the population) who hadn’t caught Covid before – a greater number than in each of the earlier waves. The fact that the infection rates appear to have been much higher in the vaccinated raises questions about the role of the vaccines in driving this.
Consider that if (as some UKHSA data suggest) the vaccinated have twice the infection rate of the unvaccinated (or more), then if the vaccinated had the same infection rate as the unvaccinated the size of the wave would almost halve in size (as most people are vaccinated). That would make it a similar size to the earlier waves – much as it was in lightly-vaccinated South Africa.
South Africa has also not seen a new surge associated with the now-dominant BA.2 variant in the way the U.K. has in recent weeks. Does this give a clue to what is going on?