Thread by @DrEricDing

There is a #B16172 variant-driven surge in parts of 🇬🇧—Bolton (near Manchester), where cases are rising fast in children & young adults. Some say this is a fewer vaccination effect in kids, some say it’s a testing surge, but it’s not clear if just testing. Need vigilance & masks.

2) some argue it’s surge testing because positivity is flat. But I’m not sure. See, if positivity % is flat, but cases rising—that means the new excess cases is real. Only if positivity dropping would it be testing driven.

Positivity flat + more cases = real increase.

https://twitter.com/jburnmurdoch/status/1395439998640574465

3) Nerding a little: So surge testing increases volume of tests, which finds more cases, yes—but if Positivity is flat, it’s then sampling same underlying case fraction—ergo actual incidence is (likely) growing. If P% dropping, then it’s oversampling and incidence likely not up.

4) But it depends on a special ratio (which @euromaestro and I discussed it a lot last year). One can think of it more precisely as the ratio of relative “case increase : testing volume increase” — if the ratio >1, then then case increase is likely true incidence increase….

5) (continued)… however if that ratio is <1 —I.e. testing volume increase outstripping case increase (aka usually positivity % down), then incidence ~not going up. This ratio works even if testing volume drops too (volume drops faster than cases drop). @euromaestro @jburnmurdoch

6) many experts in the UK are asking for school mask mandate to be reinstated. Bolton UK is seeing a likely true surge.

https://twitter.com/zubhaque/status/1395311998343258116