Thread by @BillHanage

The local school district has done a phenomenal job of keeping education going while keeping transmission minimal in the last year. We all owe everyone involved a debt of gratitude. Recent experience with omicron however, is worth sharing. It's sobering if not very surprising 1/n

The plan was to test the community before returning to school after the winter break. A day of testing, and then the positive cases would be informed and don’t come to school. This is pretty smart in normal circumstances because... 2/n

...prevalence (the number currently infected/infectious) at a given time is not the same as incidence (the number infected in a given period). This would stop a larger number of infectious folks coming into the schools. Great plan. These are not normal circumstances 3/n

The problem was the testing plan. This started with a pooled test, combining samples from 10 students. The idea was that this would mean you did not have to test all the samples, only the pools that test positive to inform whichever individual students are infected 4/n

This is only a good plan when prevalence is low. Like it was before omicron. When prevalence is high, a large proportion of pooled tests come back positive. Late last night, we all got this email 5/n

('we all' here meaning parents, I just happen to be a parent who knows things about infectious disease epidemiology, and statistics, and that the testing plan was... a bit messed up?) 6/n

If prevalence is high, you need to keep pools small, otherwise you delay actionable results. Individual results were not ready in time. And they were not ready in time for when the schools reopened this morning... 7/n

The reason for this is not clear, but willing to bet it involves the challenge of suddenly having to handle so many more tests, in numbers that obliterate your previously successful plans for test trace and isolate. Omicron has managed to outpace the first of those - testing 8/n

When schools opened today in Cambridge, we knew at least 157 people who would be able to attend them had tested +ve on Monday. But nobody knows who exactly. (in fact many parents kept their kids home. We didn’t) 9/n

This is a consequence of how quickly omicron increases. It is exactly the sort of thing I could see happening once its properties in terms of transmission became clear a month ago. It's an object lesson in how we fail to adjust to the changing pandemic 10/n

This suggests the current proportion PCR +ve in this community is ~5% (for the nerds I don’t know the Cts, I wish I did). Maybe a little more. That’s quite a lot, and it suggests that case counts are hugely undercounted which is not surprising given, you know, test shortages 11/n

This is not all bad… It is consistent with a large amount of unrecorded mild illness. Great. Problem is that most illness has been in younger people so far. Boosters will help older people a lot, but many have yet to receive them. And omicron is moving quicker than we are /end