Impact of Delta Variant and Vaccination on SARS-CoV-2 Secondary Attack Rate Among Household Close Contacts

External validation of the PRIORITY model in predicting COVID-19 critical illness in vaccinated and unvaccinated patients

“…Model performance was also comparable to the cohorts in Martínez-Lacalzada and colleagues’ original paper (C-statistic 0.823 and 0.794 in the development and validation cohorts respectively, compared to 0.794 in our cohort) ( 1 ), despite the smaller sample size, differences in demographics between study cohorts, and changes in viral variants. In our study cohorts, almost all cases were due to Delta or Omicron variants ( 11 ). The PRIORITY score remains a promising risk prediction tool, as its ease of administration permits its use on a mass scale in the community without requirement for radiographic or laboratory parameters, which is especially important during periods of disease surges and stretched healthcare capacity.…”

Vaccinating Children against COVID-19: Commentary and Mathematical Modeling

“…In light of the recent introduction of childhood vaccination against SARS-CoV-2, we forecast the effect on child and adult COVID-19 cases, hospitalizations, deaths, complications, and vaccine adverse events in two jurisdictions. Based on our mathematical model ( Tables 1 and 2 ), several observations can be made for children 5 to 11 years of age as follows: (i) high relative (percent) reduction in hospitalizations and deaths; (ii) lower relative reduction in cases because of imperfect vaccine efficacy for prevention of transmission ( 8 ); (iii) the absolute reduction in hospitalizations, deaths, and MIS-C was small, given the rarity of these events, even in unvaccinated children ( 9 ); and (iv) cases of vaccine-associated myocarditis and anaphylaxis were few ( 10 ). For adults, modest herd immunity effects were observed, with relative reduction in hospitalizations and deaths on the order of 8 to 13%.…”

“…To account for uncertainties in the vaccine efficacy, we used a multiway sensitivity analysis, varying the four key parameters (efficacy to prevent transmission, susceptibility, hospitalization, and death) over their 95% confidence interval, based on published studies ( 3 , 8 , 20 ). We assumed that each proportion followed a beta distribution.…”

SARS-CoV-2 Transmission and Prevention in the Era of the Delta Variant

“…80 On the other hand, a contact tracing study of 1,024 household contacts linked to 301 index cases in Singapore did not find a significant difference in transmission risk based on index case vaccination status (adjusted odds ratio 0.73, 95% CI: 0.38-1.40), though the confidence interval was relatively wide. 58 Finally, a contact tracing study of 471 Delta index cases and 602 contacts in the United Kingdom that collected daily upper respiratory tract samples for up to 20 days found similar secondary attack rates in contacts of vaccinated (25%, 95% CI: 15-35%) and unvaccinated (23%, 95% CI: 15-31%) index cases. 63 The discrepant findings between this study and the larger contact tracing studies may relate to its smaller sample size, intensive sampling strategy, and/or greater time since vaccination among the index cases.…”

“… 56 , 57 A household study of Delta transmission in Singapore found that older age was associated with greater likelihood of transmission, though this may relate to contact patterns within households rather than inherent host factors. 58 …”