December 15 2020
Attack Rate Analysis
Summary
The graphs below show the SARS-CoV-2 attack rates in Rhode Island and Massachusetts through November 30 2020. The attack rate is defined as the total percentage of the population infected, and it is a ‘month-early’ version of the seroprevalence time series. The analysis is based on data through Dec 6, so a patient presenting with symptoms on Dec 6 is included and counted as having been infected on Nov 30 (mean incubation period is six days). The central blue lines show the median attack rate, while the outer blue lines demark the 95% credible interval of attack rates, based on 1000 draws from the joint posterior distribution of all parameters inferred. The gray lines in the background show all 1000 trajectories. The median attack rate (and 95% CI) is shown for the end of each month in the plot. In the Bayesian inference analysis, we used case, hospitalization, and death time series, and the age-specific asymptomatic fractions in the Davies et al paper.
Attack Rates through November 30
Figure 1. SARS-CoV-2 attack rate in Rhode Island from March 1 2020 to Nov 30 2020. Model assumes that age-specific mixing rates changed in May 2020 and that the rate of ICU admission dropped in May/June 2020 (both supported by lower DIC). A prior-distribution constraint was used to ensure that Nov 30 case numbers and model output were within 10% of each other. Transmission in November was substantially higher than in September/October, with an estimated 81,000 infections occurring in November alone. At this rate, 2% of Rhode Island’s population is getting infected every week.
Figure 2. SARS-CoV-2 attack rate in Massachusetts from March 1 2020 to Nov 30 2020. Model assumes that age-specific mixing rates changed in May 2020 and that the rate of ICU admission dropped in May/June 2020 (both supported by lower DIC). A prior-distribution constraint was used to ensure that Nov 30 case numbers and model output were within 10% of each other. We assume that Massachusetts has nearly the same age-specific hospitalization probabilities as Rhode Island (in order to better infer the reporting rate). As in Rhode Island, the curve trended up sharply in November, with 3.6% of MA residents infected in November alone.