among contacts of cases that have travelled (95% CI) [secondary cases/ contacts] Secondary Attack Rate among contacts of cases that have not travelled or unknown (95% CI) [secondary cases/ contacts] 01] Unavailable [1/12] Secondary attack rates are marked as ‘Unavailable’ when count of contacts is less than 50 or count of cases is less than 20. Travellinked cases for secondary attack rates are identified positively in NHS Test and Trace data using multiple PHE sources. A case is considered as being travel-linked if EpiCell or Health Protection Teams have found evidence of international travel, their NHS Test and Trace record mentions an event associated with international travel, their NHS Test and Trace record was created after notification via IHR NFP, their contacts were traced by the international contact tracing team or they have been marked for priority contact tracing in NHS Test and Trace for reasons of travel. Some travel-linked cases may be missed by these methods and would be marked as nontravel-linked or unknown. SARS-CoV-2 variants of concern and variants under investigation 27 Secondary attack rates from NHS Test and Trace should generally be considered lower bounds due to the nature of contact tracing and testing. Data provided is for period until 1 June 2021 in order to allow time for contacts to become cases, hence case counts are lower than other sources. Cases are included in case counts if their onset or (if asymptomatic) test is during the period of study. Contacts are included in secondary attack rates if their exposure date (or onset or test of exposing case if the contact is a household contact) is during the period of study. Probable (genotyping) results are included, low quality genomic results are not. SARS-CoV-2 variants of concern and variants under investigation 28 Table 6. Secondary attack rates for all variants (excluding B.1.617 variants) (5 January 2021 to 1 June 2021, variant data as of 14 June 2021, contact tracing data as of 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 among contacts of cases that have travelled (95% CI) [secondary cases/contacts] Secondary attack rate among contacts of cases that have not travelled or unknown (95% CI) [secondary cases/ contacts] riants of concern and variants under investigation 29 Note legend from Table 5. Data provided is for period until 1 June 2021 in order to allow time for contacts to become cases, hence case counts are lower than other sources. Probable (genotyping) results are included, low quality genomic results are not. Table 7. Secondary attack rates for household contacts of non-travel cases of Alpha and Delta (29 March 2021 to 1 June 2021, variant data as of 14 June 2021, contact tracing data as of 22 June 2021) Variant Cases in those that have not travelled or unknown (with household contacts, with nonhousehold contacts) Secondary Attack Rate among household contacts of cases that have not travelled or unknown (95% CI) [secondary cases/contacts] Secondary Attack Rate among nonhousehold contacts of cases that have not travelled or unknown (95% CI) [secondary cases/contacts] Alpha 40,364 (80.4% with household, 18.4% with non-household contacts) 8.6% (8.4% - 8.8%) [7,560/87,725] 4.6% (4.3% - 4.9%) [914/20,066] Delta 32,376 (81.6% with household, 21.1% with non-household contacts) 12.0% (11.7% - 12.2%) [8,659/72,431] 6.6% (6.2% - 6.9%) [1384/21,118] Note legend from Table 5. Data provided is for period until 1 June 2021 in order to allow time for contacts to become cases, hence case counts are lower than other sources. Probable (genotyping) results are included, low quality genomic results are not SARS-CoV-2 variants of concern and variants under investigation 30 Figure 9. Secondary attack rates amongst household and non-household contacts of non-travel cases of Alpha, Delta and all others including unsequenced cases, with 95% confidence intervals. (29 March 2021 to 30 May 2021, variant data as of 14 June 2021, contact tracing data as of 22 June 2021) (Find accessible data used in this graph in underlying data.). Note legend from Table 5. Secondary attack rates are suppressed when count of contacts is less than 50 or count of cases is less than 20. Data provided is for period until 30 May 2021 in order to allow time for contacts to become cases and complete weeks to be shown. Probable (genotyping) results are included, low quality genomic results are not. SARS-CoV-2 variants of concern and variants under investigation 31 Surveillance of reinfections The COVID-19 reinfection surveillance programme aims to look at how long immunity lasts, protection against clinical disease (disease with symptoms) and protection against more severe disease. It is also important to understand whether those who become reinfected can pass the virus on to other people. SARS-CoV-2 Immunity and Reinfection Evaluation (the SIREN study) cohort monitoring The SIREN study is a cohort of National Health Service healthcare workers, including 135 sites and 44,546 participants across the UK, 35,704* in England, who remain under active follow-up with PCR testing every 2 weeks for COVID-19 by PCR. This cohort had a high seropositivity on recruitment (30% before the second wave) and is