Many people confused about level of threat posed by the Delta variant

Caution is probably best advised for those not fully vaccinated while cases continue to rise

Mixing doses of different vaccines is safe and improves the protection offered by AstraZeneca, research has found, but Ireland has yet to sanction this approach. Photograph: Getty Images

It is entirely understandable that many people find themselves confused about the level of threat posed by the Delta variant of Covid-19.

With data on the variant in its infancy, and policy responses in different countries varying hugely, it is not yet clear how concerned we should be at this latest twist to our pandemic journey.

This has given rise to frustration for many as cautious officials pull the brake on the reopening of society, while polling shows one-fifth of the population still want harsher measures introduced.

If you are not vaccinated your risk from the variant is probably little different from the risk posed by the virus up to now. There is some data from the UK indicating an increased rate of hospitalisation among Delta variant cases, but this is as likely due to the wider surge in cases as any increase in lethality on the part of the virus.


Vaccination provides excellent protection against infection and serious illness from the variant, but the level of this protection varies according to whether a person is fully or partially vaccinated and, to a lesser extent, between different vaccines.

Again, because the variant is so new real-world data on it is thin on the ground and subject to caveats. What studies there are tend to be small and not yet peer-reviewed.

Last month an English study found the Pfizer and AstraZeneca vaccines were, respectively, 96 per cent and 92 per cent effective against hospitalisation after two doses.

We haven’t yet heard how the Janssen and Moderna vaccines perform in this respect, but most scientists believe they will hold up well in preventing serious illness.


But what about infection with the virus, which can lead in some cases to long-term effects (long Covid) and onward transmissions to others?

Here the early picture is mixed. Estimates of the performance of Pfizer in stopping symptomatic infection range from 88 per cent to, in limited preliminary results from Israel, 64 per cent.

In May an English study put the two-dose protection of AstraZeneca at 60 per cent.

The most alarming piece of research to emerge recently was the study that showed both AstraZeneca and Pfizer just 33 per cent effective after one dose. Another study found a single dose of the Pfizer or AstraZeneca vaccines “barely inhibited” the variant.

Data on the performance of other vaccines in preventing infection is generally lacking so far. And while manufacturers say they will protect well, they have also begun talking about the need for additional booster doses.

Mixing doses of different vaccines is safe and improves the protection offered by AstraZeneca, research has found, but Ireland has yet to sanction this approach.

The evidence of impaired vaccine response in the face of the Delta variant has driven policy here and in other countries, as officials struggle to ramp up vaccination so that the number of fully vaccinated people is maximised.

Immune response

To recap: not everyone will be vaccinated; vaccines don’t provide 100 per cent protection; and the Delta variant seems to be able to evade the immune response to some extent.

This adds up to more cases, mostly among younger unvaccinated people but also including breakthrough infections in the vaccinated and especially partially vaccinated.

Most of these will be mild infections but with large numbers it is inevitable that some will require treatment in hospital. Just look at Donegal this week, where the north of the county has the highest infection rates and the local hospital is the busiest in the State with Covid-19 patients.

If you are fully vaccinated you probably have very little to worry about, but for others caution is probably best advised while cases continue to rise.