Hi #epitwitter SARSCoV2 attack rate in MA thru Feb 28 is estimated at 24.6% (95% CI: 22.4%-26.3%). It’s difficult to pin this down closer than 3 to 4 percentage points, as there are still several model fits that work well. Underreporting is still at about 3 for Massachusetts. 1/ 
The Jan 31 estimates had a higher underreporting rate (2.9 to 3.3), & Dec 31 estimate was even higher, closer to 4. Estimates in summer & fall were in the 3.0 to 3.4 range. Estimating the underreporting is difficult when conditions are changing quickly. 2/
As of March 19, it looks like MA is at about “26+17”, meaning 26% naturally immune, 17% immune through vaccination, for a total population immunity of 43%. The vaccination % accounts for the 3-week delay from first vaccine dose to high IgG titers. 3/
By the end of March, this will be “27+22” assuming current vaccination rates continue. By the end of April, fingers crossed, 29+38. This doesn’t account for overlap, as some previously infected individuals will get vaccinated. But, >60% immune by late April should be possible 4/
Thanks @athutran @nathanwikle @HaiderInam @BillHanage Ephraim Hanks @lmadoff @MassDPH for support, stats, and data curation. Thanks @jennifergardy for a flexible funding approach. 5/