Thread by @CillianDeGascun

An updated thread on the Delta variant and the reasons for our concern. As any new variant emerges, there are three main issues that we consider: transmissibility; infection severity; and impact on pre-existing immunity (reinfection risk) & vaccine effectiveness 1/n

Firstly, based on available evidence, the Delta variant appears to be between 40% and 60% more transmissible than the Alpha variant, which has been dominant in Ireland for the last 5 months 2/n

When compared with Alpha, Delta exhibits an increased growth rate, an increased secondary attack rate, increased household transmission, and laboratory evidence of increased replication in biological systems that model the human airway 3/n

As such, because the Alpha variant was itself significantly more transmissible than the original SARS-CoV-2 virus, we can say that Delta is almost certainly at least twice as transmissible as the virus we experienced last summer 4/n

Secondly, if we look at infection severity, data from Scotland and England show an increased risk of hospitalisation among individuals infected with the Delta variant compared to Alpha. Infection with Delta roughly doubled the risk of hospitalisation 5/n

The case-fatality rate for Delta (0.3%) at this time appears to be lower than that for Alpha (2%). However, a large number of cases are still within the follow-up period, so we still have more to learn about the clinical course of disease with Delta infection 6/n

Thirdly, looking at immunity, neutralisation studies using convalescent sera show a reduction in neutralisation against Delta. However, this does not (yet at least) appear to be associated with an increase in reinfections among recovered individuals 7/n

However, we do see a reduction in vaccine effectiveness (VE) for Delta compared with Alpha against symptomatic infection, particularly after one dose. Although this is concerning, VE against Delta is high after two doses, and VE against hospitalisation is maintained 8/n

In the UK, Delta is the dominant variant, accounting for approximately 97% of cases. As a consequence, case numbers, hospitalisations, & deaths have increased in recent weeks. ECDC also predicts that Delta will be responsible for 90% of EU/EEA infections by the end of August 9/n

In Ireland, it’s likely Delta will become dominant much sooner than that. Based on S gene data from the Thermo Fisher assay, the proportion of cases due to Alpha in Ireland has declined from 91% in week 23 (beginning June 7th) to 45% in week 25 (beginning June 21st) 10/n

Alpha cases yield S gene negative results on this assay; Delta cases yield S gene positive results. Based on the transmissibility data described earlier, it is probable that the vast majority of our S gene positive cases (i.e. 55%) are Delta. This will be confirmed by WGS 11/n

This dramatic increase in the proportion of Delta over the last two weeks will almost certainly lead to Delta dominance by the middle of July, with a consequent increase in case numbers, hospitalisations, and mortality in the following weeks 12/n

However, in contrast to last summer, we now have very effective vaccines. The key is to protect each other through established public health interventions (hand hygiene, distancing, mask-wearing, managing number of contacts) while the vaccines take effect 13/n

Please remember that you are not fully vaccinated until 2 weeks after your second dose of ChAdOx1 Astra Zeneca vaccine, or 7 days after your second dose of Pfizer/BioNTech vaccine. Two doses are required for optimal protection against the Delta variant #VaccinesWork 14/14

Finally, just to acknowledge the great work by @PHE_uk in generating & sharing their data on Delta and other #SARSCoV2 variants in real time, and informing many of the tweets in this thread #ThankYou @meera_chand @SMHopkins and colleagues