The graphs below compare Alameda County’s seven-day average case rate from the past two months to rates in neighboring Contra Costa, Santa Clara and San Francisco counties. Contra Costa and Santa Clara, in particular, are natural comparisons to Alameda, because they have similar vaccination rates and demographic data. San Francisco, on the other side of the bay, provides an additional point of reference. All data comes from the California Department of Public Health.

The case rate curves for Alameda and Contra Costa counties are near-identical. Because the neighboring counties are similar in so many respects, if masking policy had an impact on pandemic outcomes, one would expect to see some sort of discrepancy in the graph.

San Francisco and Santa Clara had higher case rates than Alameda County throughout the current surge, including pre-mask mandate. Once the mandate was introduced, the three counties all followed the same trend line, casting doubt on whether the mask mandate did anything to curb transmission at the community level.