1/ As we observe this new #BA2 fueled wave pick up real steam, I have my doubts about the signal value of wastewater as an early indicator for rising transmission rates compared to testing.
Some data points below from Silicon Valley. 🧵
2/ In Santa Clara county in California, COVID cases are now well past the height of the Delta surge and will likely approach the peak of the first Omicron wave.
3/ Based on wastewater sampling in the area, you might have “called” the rise in late April/early May but there are some puzzling head fakes of plateaus and declines in the last week.
The data is far too noisy to understand what is happening real-time or to make predictions.
4/ Meanwhile, PCR test positivity in Silicon Valley has been increasing steadily since late March, and by early April it was pretty obvious from the rise in positivity that there was another wave underway.
5/ Our clinics at @CarbonHealth in the Bay Area are as busy as they have been since the last omicron surge with COVID+ patients seeking care, so it doesn't seem like we are in a taper/decline as the wastewater data seems to suggest.
6/ TL/DR: testing still matters
• • •
Missing some Tweet in this thread? You can try to
force a refresh
Just because the official case rates look low right now doesn’t mean that transmission isn’t rampant already. Would expect #COVID to ripple across the country in the coming weeks.🧵
2/ Lots of confident takes like this one that this surge won’t be big or that the confirmed case counts look really low right now.
3/ The problem with trying to compare case counts between prior waves and the current one is that testing behavior has fundamentally changed. IHME estimates that for every 100 infections right now, only 7% are recorded in official tallies.
2/ From Sept 2021-March 2022, 40% of people who used an antigen test used it to see if a recent exposure had infected them with COVID.
3/ In the early days of an exposure before symptom onset, antigen tests lag PCR in sensitivity by several days. In the early scramble after being notified of a close contact, the first-order priority is learning whether you have COVID, not whether you are infectious.
2/ Switzerland is going through a massive wave of infections, which are being undercounted. The public health authorities there estimate that “the current wave will probably exceed the January wave”.
3/ The UK is also going through a huge #BA2 wave right now. The official reports are unreliable due to testing winding down, but the @Join_ZOE study is recording the highest level of COVID cases *ever* right now (!).
1/ This is the calm before another #COVID storm in the U.S., which will be worse than it should be due to short-term and hopeful thinking. The #BA.2 cycle has already started here and will be in full bloom in ~2-3 weeks with a much bigger surge than anyone saw coming.🧵
2/ There are many differences between UK/Europe and the US, but almost all differences weigh against the US avoiding the impact of #BA.2
- Lower vax/booster rate
- Less testing
- Fewer COVID restrictions
- Longer period of waning immunity
3/ The trough of COVID cases in Europe occurred when BA.2 penetration reached 50-60% based on genomic surveillance (BA.2 figures from outbreak.info).