Household transmission of invasive group A Streptococcus infections in England: a population-based study, 2009, 2011 to 2013

Invasive group A streptococcal infection has a 15% case fatality rate and a risk of secondary transmission. This retrospective study used two national data sources from England; enhanced surveillance (2009) and a case management system (2011-2013) to identify clusters of severe group A streptococcal disease. Twenty-four household pairs were identified. The median onset interval between cases was 2 days (range 0-28) with simultaneous onset in eight pairs. The attack rate during the 30 days after first exposure to a primary case was 4,520 per 100,000 person-years at risk (95% confidence interval (CI): 2,900-6,730) a 1,940 (95% CI: 1,240-2,880) fold elevation over the background incidence. The theoretical number needed to treat to prevent one secondary case using antibiotic prophylaxis was 271 overall (95% CI: 194-454), 50 for mother-neonate pairs (95% CI: 27-393) and 82 for couples aged 75 years and over (95% CI: 46-417). While a dramatically increased risk of infection was noted in all household contacts, increased risk was greatest for mother-neonate pairs and couples aged 75 and over, suggesting targeted prophylaxis could be considered. Offering prophylaxis is challenging due to the short time interval between cases emphasising the importance of immediate notification and assessment of contacts.