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Another factor in Hong Kong’s outbreak is that its earlier successes may be working against it now. By keeping cases down, there are fewer people with immunity acquired through infection with SARS-CoV-2. “Therefore those people who are unimmunized are vulnerable, and there are more of them in a place like Hong Kong than here because of the lack of prior infections,” said William Hanage, an associate professor of epidemiology at Harvard University, in an email.

とある。ハーバード大准教授の見解。