(Find accessible data used in this graph in underlying data). SARS-CoV-2 variants of concern and variants under investigation 24 Secondary attack rates This section includes secondary attack rates for traveller and non-traveller cases, and separate household contact rates, including new analysis of rates for household and nonhousehold contacts of non-traveller cases over time for Delta and Alpha variants. Secondary attack rates are based on positive tests amongst contacts named to NHS Test and Trace by an original case identified with a confirmed or probable variant of concern or variant under investigation. Variant cases are identified using confirmed (sequencing) results supplemented with probable (genotyping) results as of 14 June 2021, and exclude LQ-HRG results. Secondary attack rates are shown for cases with and without travel history. In non-travel settings, only close contacts (household members, face-to-face contact, people within 1 metre of the case for 1 minute or longer, or people within 2 metres for 15 minutes) named by the original case are included. In travel settings, the contacts reported are not restricted to only close contacts named by the case (for example, they may include contacts on a plane linked by additional contact tracing efforts), leading to likely deflation of secondary attack rates amongst travellers compared to non-travellers. In addition, people recently returning from overseas are subject to stricter quarantine measures and may moderate their behaviour towards contacts. Travel history suggests, but does not confirm, where infection of the original case may have occurred. Table 5 shows the secondary attack rates for Delta compared to the other B.1.617 variants and Alpha. The time period of study for secondary attack rates has been restricted to the period 29 March 2021 to 1 June 2021, to capture recent social restrictions and vaccination levels. A reduction in secondary attack rate for non-travel cases with Alpha is observed in this shorter period when compared to Table 6 covering 5 January 2021 to 1 June 2021. Secondary attack rates for contacts of cases with Delta and no travel history are higher than those for contacts of cases with Alpha and no travel history: 10.7% (95% CI 10.5% to 10.9%) compared to 7.9% (95% CI 7.7% to 8.0%). Estimates of secondary attack rates for contacts of those that have travelled with variants of concern or variants under investigation were all considerably lower than those that have not travelled, due to the difference in contact definition. Secondary attack rates for contacts of travel cases with Delta were higher than those for travel cases with Alpha. Table 6 shows the secondary attack rates for variants (excluding variants of the B.1.617 lineage, that is Delta, Kappa, VUI-21APR-03) for the period 5 January 2021 to 1 June 2021. Secondary attack rates for contacts of non-travel cases with VUI-21MAY-01 were lower than for contacts of non-travel cases with Alpha over this time. All other secondary attack rates for contacts of non-travel cases with the remaining variants of concern or SARS-CoV-2 variants of concern and variants under investigation 25 under investigation are not significantly different from Alpha. Estimates of secondary attack rates for contacts of those that have travelled with variants of concern or variants under investigation were all considerably lower than those that have not travelled, due to the difference in contact definition. Table 7 shows the secondary attack rates amongst household and non-household contacts of non-travel cases with Delta and Alpha. The time period of study for secondary attack rates has been restricted to the period 29 March 2021 to 1 June 2021 as in Table 5. Secondary attack rates are higher amongst household contacts than non-household contacts of non-travel cases with both variants and higher for contacts of non-travel cases with Delta than Alpha; this is consistent with Table 5. Figure 9 shows the secondary attack rates amongst household and non-household contacts of non-travel cases with Delta and Alpha over time for the period 29 March 2021 to 30 May 2021, with 95% confidence intervals. The fall in secondary attack rate amongst household contacts of cases with Delta in previous weeks has now levelled, with an estimate of 12.0% (95% CI 11.6% to 12.4%) for exposure events in week commencing 24 May 2021. Over the period presented, secondary attack rates for both household and nonhousehold contacts of cases with Delta remain higher than for Alpha (or other cases). A peak in secondary attack rates from cases with Delta was seen in both household and non-household contacts exposed during the week commencing 26 April 2021. Secondary attack rates amongst household contacts of cases with Alpha also peaked in week commencing 26 April, though the increase up to that week and decline since that week are modest. SARS-CoV-2 variants of concern and variants under investigation 26 Table 5. Secondary attack rates for Kappa, Delta and VUI-21APR-03 (B.1.617.3), presented with Alpha, time restricted for comparison (29 March 2021 to 1 June 2021, variant data as at 14 June 2021, contact tracing data as at 22 June 2021) Variant Cases in those that have travelled (% with contacts) Cases in those that have not travelled or unknown (% with contacts) Case proportion that have travelled Secondary Attack Rate