What is the Delta variant and how much does vaccination protect against it?

Explainer: Chief medical officer warns a fifth of Covid-19 cases in State now Delta variant

The Delta variant is 50 per cent more infectious than the Alpha, or Kent B117 variant. File photograph

While a strong sense of optimism remained about Ireland’s progress against the virus up until last week, there appeared to be a shift in tone when the chief medical officer Dr Tony Holohan warned on Monday that the Delta variant was estimated to account for a fifth of new Covid-19 cases.

The variant, which originated in India, is 40 to 60 per cent more infectious than the Alpha strain (previously known as the UK or Kent B117 variant), which in turn is 50 per cent more transmissible than its Wuhan predecessor. Alpha is still the dominant force in Ireland, but in the space of one week Delta has increased from five per cent to 20 per cent of new cases here.

The World Health Organisation (WHO) executive director Mike Ryan noted on Monday that the Delta variant is “faster”, “fitter” and “will pick off the more vulnerable more efficiently than previous variants”.

Last Thursday National Public Health Emergency Team (Nphet) officials said all indicators were moving in the right direction, with cases, hospital admissions and deaths reducing. However, the black cloud of the Delta variant loomed as a “clear threat”, Dr Holohan had said.


By then there had been 188 coronavirus cases sequenced to the Delta variant, and Professor Philip Nolan said most of these were being intercepted at or soon after their point of entry into the State. Over 80 per cent of these cases were in Dublin, where their number had reduced, officials said.

A “concerning increase” in the variant’s presence in the new cohort of cases in the State over the past week is following a similar pattern that is being seen across a number of European countries, Dr Holohan warned on Monday.

A number of outbreaks associated with the strain have been reported in Ireland over the last seven days, with a cluster of cases in Athlone “probably” of the Delta variety, according to the Director of Public Health Midlands Dr Una Fallon. There, public health officials are currently trying to trace people who socialised on the banks of the River Shannon, with Dr Fallon expressing concern such an outbreak could “stretch far and wide”.


The vaccination rollout is developing a protective shield around large swathes of the population, and full vaccination is effective against this new strain. Close to two-thirds of the adult population have received at least one dose, while about 35 per cent of adults were fully vaccinated, according to the HSE’s vaccination programme lead, Damien McCallion. The programme is now hitting its peak, with more than 340,000 vaccinations administered last week and over 300,000 doses to be given this week.

Data from Public Health England suggests full immunisation provides similar levels of protection against hospitalisation with Delta as it does against the Alpha variant. The analysis suggests the Pfizer/BioNTech vaccine is 94 per cent effective against hospital admission after just one dose, rising to 96 per cent after two doses. The AstraZeneca vaccine is 71 per cent effective against hospitalisation after just one dose, increasing to 92 per cent after two doses.

However, much of the concern among health officials relates to the potential for the disease to spread among young, unvaccinated cohorts, as well as those who are only partially inoculated. Studies indicate that those who have received only one dose have less protection against symptomatic infection from the Delta strain than earlier variants.

Minister for Health Stephen Donnelly said Covid-19 cases were rising only in one age cohort: people aged 19-24 years, a group that is largely unvaccinated. Dr Holohan also warned anyone not yet fully vaccinated to stick to all public health guidelines.

Within three weeks of the first shot of both the AstraZeneca and Pfizer vaccines each offers just 33 per cent protection against Delta symptomatic infection, according to a Public Health England study. After two jabs, AstraZeneca’s protection against symptomatic disease from the Delta variant rises to 60 per cent (compared to 66 per cent against Alpha), while Pfizer is 88 per cent effective against symptomatic disease from the Indian variant two weeks after the second dose (down from 93 per cent against Alpha).


As Dr Holohan pointed out, Delta has been the dominant Covid-19 strain in the UK for a number of weeks. Concern that over 90 per cent of cases were of this new variety led to a delay in the British government’s reopening plans.

However, on Tuesday the UK’s health secretary Matt Hancock spoke encouragingly about the slowing growth in cases in Britain. There were 33,630 new Delta cases confirmed in the UK last week, a 79 per cent increase that brought the total number of cases associated with the variant there to 75,953. The number of hospitalisations, he told Sky News, is “not rising very quickly” and the number dying with the disease “remains very, very low”. Approximately 78 per cent of adults in the UK have received one Covid-19 vaccine dose, while 47 per cent are fully vaccinated.

Tighter quarantine and testing arrangements were imposed from last week on unvaccinated travellers to the Republic from Britain, but there were no restrictions on travel to Northern Ireland. About a quarter of all cases in Northern Ireland were associated with the Delta variant as of last Thursday, but there have been clusters of the mutation recorded in recent days, including a spike in Derry city. Chief medical officers on both sides of the Border met last Friday to discuss concerns over the Delta variant. There is particular concern among officials about importation of Delta cases into the Republic via the northwest.

While the Delta variant is more transmissible, a spokeswoman for the Department of Health confirmed on Tuesday that current public health advice is the same for all Covid-19 variants.

She said anyone who is not yet fully vaccinated should continue to follow all public health advice, including avoiding crowds, limiting contact with others, refraining from meeting up indoors, and continuing to work from home where possible.