Only in the presence of social distancing or other countermeasures.
20% antibodies means that you could have an R = 1.25 and still get an effective R < 1.0, preventing further onwards transmission. R0 estimates for COVID-19 ranged from 2.5-6, so by itself 20% of the population with antibodies wouldn't do anything (it'd reduce R to 2-4.8, which is still pretty quick exponential spread, faster than the flu). However, R in the U.S. around the time of the protests was measured at about 1.07, because it wasn't fully opened up yet and many people are staying home out of fear. Under those conditions NYC gets an effective R = 0.85 (epidemic dies out), while a state with 2% antibodies has an effective R = 1.05, which is still positive (albeit slow) exponential spread.
I suspect it's a combination of the lax social distancing requirements + lack of immunity. NYC would still be experiencing exponential spread without existing immunity, but it's only because they're still locked down that the level of immunity they have can prevent an epidemic.
20% antibodies means that you could have an R = 1.25 and still get an effective R < 1.0, preventing further onwards transmission. R0 estimates for COVID-19 ranged from 2.5-6, so by itself 20% of the population with antibodies wouldn't do anything (it'd reduce R to 2-4.8, which is still pretty quick exponential spread, faster than the flu). However, R in the U.S. around the time of the protests was measured at about 1.07, because it wasn't fully opened up yet and many people are staying home out of fear. Under those conditions NYC gets an effective R = 0.85 (epidemic dies out), while a state with 2% antibodies has an effective R = 1.05, which is still positive (albeit slow) exponential spread.
I suspect it's a combination of the lax social distancing requirements + lack of immunity. NYC would still be experiencing exponential spread without existing immunity, but it's only because they're still locked down that the level of immunity they have can prevent an epidemic.