The district of Heinsberg was the first and hardest-hit region in Germany (250,000 inhabitants, 1615 infections, 55 deaths). In early March, scientists launched a wide-ranging household study in the community of Gangelt, turning the region into a COVID-19 "laboratory" for further predictions. In mid-April, the leading virologist Hendrick Streeck announced that the infection rate was at 15 percent, indicating the beginning of "herd immunity." He was criticized by Christian Drosten, a well-known virologist based in Berlin, for early publication of the findings, which he considered too vague.
"A reality check on the math': What's ahead for California and Covid-19." Guardian, 8 April 2020
https://www.theguardian.com/us-news/2020/apr/07/california-covid-19-whats-next-outbreak-lockdown
"California officials this week described a “slow and steady increase” in confirmed coronavirus infections, predicting that the peak of the state’s outbreak will occur in mid to late May. Andrew Noymer, an associate professor of public health at the University of California, Irvine, explained what was behind the state’s projections – and what Californians can expect next. ... Earlier projections estimated California would see its peak in mid-April. ... Vaccination would be a huge game-changer, but that looks 12 months away or more. So we all need to steel ourselves for doing this at least up until 1 June and potentially longer."