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Measles transmission during a large outbreak in California
Lee Worden1, Sarah F Ackley2, Jennifer Zipprich3
1Francis I. Proctor Foundation, University of California, San Francisco, CA, USA.
Abstract:
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.
Insights
Measles transmission significantly declined during the 2014-2015 California outbreak, linked to Disneyland. This study analyzed case data to understand transmission dynamics and generation timing.
Area of Science:
- Epidemiology
- Infectious Disease Dynamics
- Public Health
Background:
- A significant measles outbreak occurred in California during 2014-2015, with links to Disneyland theme parks.
- This outbreak garnered international attention and prompted revisions in California's vaccine policies.
Purpose of the Study:
- To estimate time-varying transmission rates during the 2014-2015 California measles outbreak.
- To probabilistically estimate the generation membership of cases within the outbreak.
- To assess the sufficiency of outbreak size and duration in distinguishing temporal decline from constant transmission.
Main Methods:
- Utilized dates of symptom onset for California measles cases.
- Incorporated known epidemic links to model transmission dynamics.
- Employed probabilistic methods to estimate case generation membership.
Main Results:
- Found a statistically significant decline in measles transmission over the course of the outbreak (p=0.012).
- Transmission rate estimates by day or generation may overestimate the temporal decline.
- Outbreak size and duration were sufficient to distinguish temporal decline from time-invariant transmission (p=0.014).
Conclusions:
- Measles transmission rates demonstrably decreased during the 2014-2015 California outbreak.
- Caution is advised when interpreting immunity estimates derived solely from this single large outbreak.
- Further research is necessary to elucidate the specific causes of observed temporal declines in transmission.
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