Threat Assessment Brief: Implications for the EU/EEA on the spread of the SARS-CoV-2 Delta (B.1.617.2) variant of concern

Based on the available evidence, the SARS-CoV-2 Delta (B.1.617.2) variant of concern (VOC) is 40-60% more transmissible than the Alpha (Β.1.1.7) VOC and may be associated with higher risk of hospitalisation. Furthermore, there is evidence that those who have only received the first dose of a two-dose vaccination course are less well protected against infection with the Delta variant than against other variants, regardless of the vaccine type. However, full vaccination provides nearly equivalent protection against the Delta variant.

Based on the estimated transmission advantage of the Delta variant and using modelling forecasts, 70% of new SARS-CoV-2 infections are projected to be due to this variant in the EU/EEA by early August and 90% of infections by the end of August.

There is a well-documented age-risk gradient for SARS-CoV-2, where older age groups and those with underlying co-morbidities are more likely to be hospitalised or die due to COVID-19. In a scenario of 50% gradual reduction of non-pharmaceutical intervention (NPI) measures by 1 September, SARS-CoV-2 incidence is expected to increase in all age groups, with the highest incidence in those <50 years.

Modelling scenarios indicate that any relaxation over the summer months of the stringency of nonpharmaceutical measures that were in place in the EU/EEA in early June could lead to a fast and significant increase in daily cases in all age groups, with an associated increase in hospitalisations, and deaths, potentially reaching the same levels of the autumn of 2020 if no additional measure are taken.

Risk assessment

Evidence accumulated since the first threat assessment brief on the emergence of the SARS-CoV-2 Delta variant in India, published 11 May 2021, resulted in the Delta variant being upgraded from a Variant of Interest (VOI) to a VOC. The assessment of the risk for infection to unvaccinated and partially vaccinated individuals from the Delta VOC in the EU/EEA has also increased.

Considering the very high probability of the Delta VOC becoming the dominant variant in the EU/EEA:

• The overall risk of SARS-CoV-2 infection related to the expected increase in circulation of the Delta VOC for the general population is considered to be low for fully vaccinated sub-populations and high-to-very high for partially or unvaccinated sub-populations.
• The overall risk of SARS-CoV-2 infection related to the expected increase in circulation of the Delta VOC for vulnerable population is considered to be low-to-moderate for fully vaccinated sub-populations and very high for partially or unvaccinated sub-populations.

Since ECDC’s most recent risk assessment published on 10 June, and given the expected future predominance of the Delta variant, the risk has increased for countries in all epidemiological situations. Without continued application of NPI measures and further rapid rollout of full vaccination, sharp increases in new infections, hospitalisations and deaths may be observed.

Options for response

Full vaccination of all groups at increased risk of severe COVID-19 should be achieved as early as possible to reduce the risk of hospitalisations and deaths. In order to achieve maximum protection in the shortest time possible, it is recommended that individuals at highest risk of severe outcomes for SARS-CoV-2 receive a second vaccine dose in the shortest possible interval following the administration of the first dose.

The continuation of vaccination rollout at current levels is crucial in order to keep the incidence levels at manageable levels, and further acceleration of vaccination rollout, including achieving higher levels of vaccination coverage, could have a substantial impact on decreasing incidence, hospitalisations and deaths, particularly in older age groups.

Non-pharmaceutical interventions should be maintained at a level sufficient to contain community transmission of the Delta VOC until greater shares of the population are fully vaccinated, in order to avoid a resurgence of cases with a possible increase in hospitalisations and mortality.

Genomic surveillance of currently circulating variants (including weekly representative samples of sufficient sample size and targeted samples from special settings and populations) is of high importance for early detection and monitoring of emerging SARS-CoV-2 variants. Member States who require support to reach sequencing targets can use ECDC services for sequencing of SARS-CoV-2.