[Black Agnes]

If that’s true, howcome some neighborhoods in NYC have 70%+ seropositivity?

Did they not have t-cells?

[ETCM]

T-cells do not prevent infection. They do not prevent an antibody response. In fact, they are key to the antibody response process. They control viral transcription while the body's other defenses build, by killing infected cells and activating the B-cells that produce neutralizing antibodies. It's still early, and more studies are needed, but they believe with SARS-CoV2 the antibodies fade quickly while the t-cell response remains longer term. So the folks that were sick back in Mar/Apr and no longer have antibodies might still have some significant level of immunity. Here is a link that has nothing to do with SARS-CoV2, but explains the t-cell response and how t-cells activate b-cells.

B Cells: The Antibody Factories of the Immune System

If you already have these memory t-cells, your immune system has a huge advantage over the virus. If you do not, the virus spreads uncontrolled for several days while your immune system gets started. This is actually "normal" for many respiratory viruses, including SARS-1. Here's a research paper from 2018 that discusses memory t-cells being critical to response and immunity in respiratory viruses in general, and how they can provide cross protection from other viruses:

The CD8 T Cell Response to Respiratory Virus Infections

A quote from the conclusion:

...robust memory CD8 T cell responses efficiently reduced lung viral titers in the absence of neutralizing antibodies following RSV, IAV, or SARS secondary infections. An important property of CD8 T cells is that they often recognize conserved viral proteins, allowing for cross-protection between different virus strains.