If San Francisco can give doses to 5,000 new people per day -- a number that's certainly possible once supplies increase -- then the city will reach 85% immunity in 96 days. That's May 30.
And by day 119, or June 22, 100% of eligible San Franciscans will have acquired some immunity to COVID-19, whether that's through receiving one dose, two doses or getting the disease itself.
Between the emergence of new coronavirus variants, unreliable vaccine supplies and uneven access to the doses available, it may take months or even years longer than anyone would like to hit herd immunity. It's possible California, the nation and the world may never get there. [...]
Assuming herd immunity is achievable, there are several variables that could slow it down. One unknown is how long immunity lasts after people are vaccinated. Some infectious disease experts fear immunity could start to wane in just a few months, which would mean the first round of people to be vaccinated will need boosters before the final groups get a single dose. But most experts believe immunity probably lasts longer than that, maybe years.
The coronavirus variant originally found in Manaus in Brazil and now detected in six cases in the UK was able to evade 25% to 61% of the protection people in the Amazonian city had after a first bout of Covid, researchers say. [...]
Manaus, the largest city in the Amazon basin, suffered a first wave of Covid in April/May. Studies of blood donors suggested 66% of people had antibodies against the virus in July and 76% by October, which would have been expected to give them immunity. [...] It spread rapidly: within eight weeks, it was implicated in 87% of cases.