Vaccines are highly effective


Sir, – As an Irish person living abroad, I have greatly appreciated your detailed and careful coverage of the Covid pandemic in Ireland, including your most recent article (“Covid-19: 1,189 new cases confirmed in State with 23 patients in ICU”, News, July 22nd).

However, the article contains a statistic that could easily be misinterpreted by readers. This is that a significant fraction of those currently hospitalised with the virus (18 out of a total of 86 nationally) have been fully vaccinated. This figure should not be taken as an indication that vaccines don’t work well against the Delta variant.

In fact, this is an example of what statisticians call “sampling bias”, which arises from comparing groups that should not be directly compared. In Ireland, as the article notes, the vaccinated population is very heavily skewed towards those at highest risk of severe Covid, in particular the elderly. By definition, this population will suffer a disproportionate number of cases of severe Covid. This may happen even if, as recent studies suggest, vaccines are highly effective.

In the US, where I live, the Delta variant accounts for a large majority of cases, as in Ireland. Around 50 per cent of the population are fully vaccinated, as in Ireland. Unlike Ireland, however, we suffer from widespread vaccine hesitancy, so high-risk groups here in the US have much lower rates of vaccination. In this scenario, we can see just how good the jabs are: the unvaccinated currently account for 97 per cent of hospitalisations and 99.5 per cent of deaths from Covid.

It is worth emphasising over and over again: vaccines provide excellent protection against severe illness and death from all widespread Covid variants, including Delta. – Yours, etc,


Department of Molecular

and Cellular Biology,

Harvard University,

Cambridge, Massachusetts.