I’ll guess that some of it is because within-home transmission was a surprisingly high fraction of Rt in late March and the first few days of April. That will involve Rt close to 1 for transmission to adults in a typical household, until immunity effects inhibit a good deal of within-home transmission. I’m also guessing that infections are mostly going undetected in children.
Limits on test availability could affect reporting of COVID-19 deaths. I expect that some hospital deaths in March weren’t tested for COVID-19. Maybe also some were swabbed, and testing of those swabs was given low priority, with the result that some deaths are being reported with significant delays. There’s been some recent confusion over Pennsylvania’s reported deaths which suggests that standards have been changing for how deaths are counted, and that seems to be adding a few more reported deaths now than would have been reported a month ago.
It does seem likely that the measures that have been taken do a lot more to inhibit between-household transmission than within-household transmission, so there could be a lag while within-household transmission works itself out before an exponential decline in new cases/deaths becomes evident.
And if there’s a testing backlog, and deaths are only recorded as COVID-19 deaths once a sample is finally tested, that would also introduce a lag. This seems like it must vary a lot with locale, though...
I’ll guess that some of it is because within-home transmission was a surprisingly high fraction of Rt in late March and the first few days of April. That will involve Rt close to 1 for transmission to adults in a typical household, until immunity effects inhibit a good deal of within-home transmission. I’m also guessing that infections are mostly going undetected in children.
Limits on test availability could affect reporting of COVID-19 deaths. I expect that some hospital deaths in March weren’t tested for COVID-19. Maybe also some were swabbed, and testing of those swabs was given low priority, with the result that some deaths are being reported with significant delays. There’s been some recent confusion over Pennsylvania’s reported deaths which suggests that standards have been changing for how deaths are counted, and that seems to be adding a few more reported deaths now than would have been reported a month ago.
Those are both good points.
It does seem likely that the measures that have been taken do a lot more to inhibit between-household transmission than within-household transmission, so there could be a lag while within-household transmission works itself out before an exponential decline in new cases/deaths becomes evident.
And if there’s a testing backlog, and deaths are only recorded as COVID-19 deaths once a sample is finally tested, that would also introduce a lag. This seems like it must vary a lot with locale, though...