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How many people on average will a Delta positive infect? What about transmission from a breakthrough infection?

Question: Do we have a sense of, on average, how many people a Delta variant positive person can pass along the virus to? What about transmission from breakthrough infection?

Answer: Good question!  Here’s a three part answer:

  1. Reproduction number (R0). In the absence of public health measures and vaccines, each person infected with Delta would likely spread it to 5 or 6 others (for more on R0, see our Q&A of 8/31).  
  2. Secondary attack rate. A related concept is the secondary attack rate– the proportion of individuals who are exposed to the primary (index) case and become infected themselves. Of course, secondary attack rate is also context dependent. A recent meta-analysis of 87 studies on this topic found that the estimated household secondary attack rate was 19%. On average, 19% of household contacts become infected. A more narrow analysis from Public Health England (September 17, Technical Briefing 23) found that Delta-related secondary attack rates among household contacts have risen in recent weeks with an estimate of 11.4% of household contacts becoming infected (95% CI: 11.2% to 11.6%).  Secondary attack rates among non-household contacts have remained steady over recent weeks at 4.9% (95% CI: 4.6% to 5.2%) Figure 1.  
  3. Index case vaccination status. Unfortunately, Public Health England has not disaggregated its data by vaccination status of the index case.  Additionally, since we’re still early into the Delta dominance (science-wise), we don’t have many published studies directly related to answering the question on what the household secondary attack rate is stemming from Delta-related breakthrough infection.  Based on the studies that we do have, we know that if the index case is vaccinated (e.g. experiencing a breakthrough infection), the secondary attack rate in households is much lower.  In fact, studies from earlier this summer (e.g. not Delta-specific) show that vaccination halves household secondary attack rates (see these studies from NEJM, EuroSurveillance, and this pre-print).Since these studies are not Delta-specific, it’s unknown whether the findings would be consistent now, which is why we need more research.  That said, we would expect secondary attack rates to be lower with vaccination, even in the context of Delta.  

Figure 1. Secondary Attack Rates among Household and Non-household contacts over time, Delta and Alpha Variants (from Public Health England)