Measles transmission during a large outbreak in California

•An exceptionally large outbreak of measles in 2014-2015 provides an opportunity to assess patterns of transmission.•California contact investigations provided transmission sources for many of the cases.•Using estimation of number of secondary cases arising from each case, transmission rates are found to have fallen during the course of the outbreak, even taking into account that they appear to fall when transmission is actually constant.•The same result is found using only the total size and duration of the outbreak. A large measles outbreak in 2014–2015, linked to Disneyland theme parks, attracted international attention, and led to changes in California vaccine policy. We use dates of symptom onset and known epidemic links for California cases in this outbreak to estimate time-varying transmission in the outbreak, and to estimate generation membership of cases probabilistically. We find that transmission declined significantly during the course of the outbreak (p = 0.012), despite also finding that estimates of transmission rate by day or by generation can overestimate temporal decline. We additionally find that the outbreak size and duration alone are sufficient in this case to distinguish temporal decline from time-invariant transmission (p = 0.014). As use of a single large outbreak can lead to underestimates of immunity, however, we urge caution in interpretation of quantities estimated from this outbreak alone. Further research is needed to distinguish causes of temporal decline in transmission rates.