The truth about claims Covid jabs 'driving massive infections' among vaccinated Scots

Sometimes the numbers are simply so confusing and counterintuitive that sharing them can do more harm than good.

That at least has been the weary conclusion of Public Health Scotland scientists this week following the decision to stop publishing its tables showing Covid case rates, hospitalisations and mortality by vaccination status after they drew the attention of vaccine sceptics.

A former advisor to the Trump administration, epidemiologist Paul E Alexander, went so far as to tell a US Senate committee hearing that Scottish figures demonstrate "conclusively" that second and booster doses are "driving massive infections in the vaccinated".

READ MORE: Trump advisor cities Scottish data in US Senate to claim 'vaccines not working'

To be clear, they are not; but you could be forgiven for thinking that that is exactly what the statistics show - if you take them at face value, anyway.

So what's really going on?

Ever since PHS first began publishing the tables a few months into the rollout they have served as a proxy for vaccine effectiveness - even if that never was their real purpose - because week after week it was clear that unvaccinated individuals were consistently more likely than their fully-vaccinated counterparts to end up infected, in hospital, or dead.

There were two particularly important aspects to the data.

First, it is "age-standardised", which enables the vaccinated and unvaccinated to be compared fairly despite the fact that young people were the last to be offered the jags, and the least likely to take them, but also more likely to be in good health and thus less vulnerable to Covid complications.

Secondly, providing rates "per 100,000" means the figures are not skewed by the fact that, as more and more of the population becomes vaccinated, the number of vaccinated people in hospital with Covid gradually - and inevitably - outnumbers the unvaccinated.

Up until December the tables provided reliable reassurance that the vaccines were doing as intended: reducing - but never eliminating - the risk of infection, hospitalisation and death.

Then Omicron arrived, and something very puzzling began to occur.

First the case rates between unvaccinated and double-vaccinated converged, then the latter edged ahead.

By the week leading up to Christmas Eve, it was even stranger: the case rate in the unvaccinated - at 541 per 100,000 - was notably lower than even the boosted group (751 per 100,000) and dwarfed by the double-vaccinated on 1,328 per 100,000).

The most recent weekly case rate table, published on Wednesday (Source: Public Health Scotland)

It wasn't a blip either. The pattern continued, week after week, up until the most recent publication on Wednesday with case rates of 341 per 100,000 in the unvaccinated compared to 550 and 528 per 100,000 respectively in the double-vaccinated and boosted.

England is also seeing higher case rates in the vaccinated (NB: England data not age-standardised; Source UKHSA)

Scotland is not unique - England and Ontario in Canada are reporting exactly the same phenomenon.

In Scotland's case, Omicron exposed a weakness in the data: the size of the unvaccinated population had long been overestimated.

READ MORE: 'It may be fine - but it's a gamble' - Should Scotland end self-isolation too?

If Scotland's total population is counted based on the number of people registered with GPs here, it comes out 8% higher than official (more accurate) estimates from the National Records of Scotland.

In order to track people who test positive according to their vaccination status, however, GP health records are required.

But then things become even worse: when GP registration is used to estimate the percentage of unvaccinated Scots aged 12-plus, it comes out at 16% compared to 8% using NRS figures.

Overestimating the size of the unvaccinated population matters much less when prevalence is low, but becomes increasingly problematic by the time one in 25 people are infected.

Table showing effect of denominator (estimate of total unvaccinated population) on case rates

Supposing the actual case rate were two per 100, for example, it would be be halved to one per 100.

But if the prevalence quadruples - as it has compared to a year ago - the gulf between the correct and incorrect figure widens and you end up with an actual rate of eight per 100 coming out as four per 100, all because the denominator (the unvaccinated population total which cases are divided against) is so out of whack.

The problem is that GP registrations count "ghost" patients: those on practice lists, but no longer resident in Scotland. People recently vaccinated in Scotland, on the other hand, will almost all still be living here.

Separately, there is also evidence that vaccinated individuals are more likely to get tested and, in particular, to self-report a positive lateral flow result - a bias which has become potentially more consequential recently.

READ MORE: 'Weird' data shows Scotland's Covid case rates lowest in the unvaccinated

Meanwhile, Omicron also resulted in the hospitalisation rate among the double-vaccinated pulling ahead of the unvaccinated in December, for the first time, for different reasons.

Omicron arrived just as second dose vaccination was waning. Those double-vaccinated individuals ending up in hospital were overwhelmingly over 70 (in younger age groups, hospitalisation rates remained highest in the unvaccinated), and overdue for their booster.

Hospital admission rates in the most recent week. Mortality and hospitalisation rates have remained consistently much lower in the boosted group throughout the Omicron wave

The fact that these elderly people had not yet had a booster mostly reflects complex health issues and frailty, compared to unvaccinated and boosted people of the same age. Some will have been hospitalised for these pre-existing conditions only to contract Covid on the wards.

"We don't think it's in the public interest really now to publish [the data] because it's so misleading," one PHS official told the Herald.

The tables will be published quarterly now, instead of weekly.

Yet withdrawing the statistics in a bid to to prevent them becoming fodder for antivaxxers also has the effect, surely, of making them seem like some sort of embarrassing smoking gun to be hidden?

"I'm sure we'll get accused of a cover up," added the official.

"I am worried about that."