That’ll explain the decline in hospitalizations and deaths but not the number of new cases, especially among the young. I think herd immunity is starting to kick in, and yes vaccination campaign is helping but it’s not the only thing.
Look more closely at today’s stats. 13 states made no report. Likely since today was Sunday. Tomorrow’s report for those states will likely include the numbers from Sat as well as Sunday. 13 states I would anticipate an increase of at least 5 to 6K over the 36K listed. Still good but not as good.
Herd immunity for a disease with an R naught (R0) value of 2.5 kicks in at 60%. The formula for calculating herd immunity is 1-1/R0. The R naught of Covid-19 is between 2 and 3, so I use the value of 2.5.
For the US, that means around 198 million people would have to be immune for herd immunity to set in. We don't know how long immunity from the illness lasts; some studies suggest that it is a few months only. In any case, the number of people who have recovered together with those who have been vaccinated is not even close to 198 million. Any decreases in cases and deaths at this point are still due to measures to slow spread, such as masks and social distancing.