In June, after reopening, #SARSCoV2 #COVID19 cases in Florida began to rise. Hospitalisations & deaths, however, stayed low.

Perhaps it just wasn't so bad after all? Perhaps something had changed?

We see similar trends in Europe now. So what happened in Florida?
Let's see

Something was going on in the new cases in June - noted in the media across much of the US South: new cases were mostly in the young. Note the change in proportion of tests in 20-34 yo (orange) over time

Huh. Why might that be?

@nataliexdean excellently explains 3 options:
1- More & more general testing (more likely to test any less symptomatic young ppl)
2- Elderly people being /more/ cautious
3- Young people being /less/ cautious

In reality, it was likely all 3 things, as @nataliexdean shows with @trvrb's figure. Including a real increase in infections in younger age groups.

While most cases are in younger people, things feel very 'safe'. So, are we in the clear to just wait-&-see? Maybe things will stay this way!

But do we really expect that cases will only stay in young people, and not spread to other age groups?

Florida 'waited-&-saw', so we can have a look at what happened.

@zorinaq's heatmap shows this well: a slow burn in young age groups begins to spread upward to older individuals - those with higher risk of severe outcomes.

@nataliexdean spotted this too - and we can see it clearly in the data. Note how after June, the proportion of cases in 20-34 yo (orange) decreases (lower graph) as cases rise & peak (upper graph):

On top of this - remember, deaths lag!

Florida (along with many other southern US states with similar patterns) has had a severe outbreak, & had to reverse reopening to contain it.

After having cases under control for so long, why did they fail to act? Why wait?

Wishful thinking, perhaps. @zorinaq fantastic chart below is annotated with 1 (popular but wrong) theory - T-cell protection?
But more generally, as we've seen many times in this pandemic already: an inability to imagine it could get that bad, here.

Now - to today. Europe. In many countries, cases are rising - but hospitalisations & deaths are not! We see graphs like the below for Spain & Switzerland.

Perhaps it's not so bad after all...?

Unfortunately, we see a similar trend to Florida in June: Rising cases among young people. For Switzerland, @erwinheim's tables make this clear - but we can plot the data too.

Here I plot #SARSCoV2 tests by age group over time in Switzerland. Spiky bits in end-May to mid-June show low case numbers. But compare the percent that's orange & light orange (20-39) in spring - and now.

Distinctly higher proportion of cases in young people.

If we'd like to compare a little more closely to Florida's data, we can split up the age categories & change colors to approximate it (Florida on right):

Their cases didn't stay in young people. Will Europe's? Probably not.

One question I've been getting a lot in the past few days is - maybe the #SARSCoV2 virus has changed? Maybe the version we have now is mutated to be less severe?

We can look into this, too.

Here, I highlight Swiss sequences on @nextstrain - but many other European countries show the same thing.

The whole tree shows the entire diversity of the virus. The x-axis is time - dots are #SARSCoV2 sequences plotted when they were taken.

Notice that early in the pandemic (Mar-May) Switzerland already had sequences from almost the entire available viral diversity (scattered top-to-bottom on tree).

But the most recent sequences are similar - they're also from all over the tree. This indicates 2 things...

1st - Overall diversity of viruses hasn't changed. After Mar, we don't see a variant becoming dominant in Europe that could be carrying such a mutation

2nd - So, a 'less severe' mutation would have to arise in all/most of those independent lineages for us to see an effect

So no - we see no evidence that the virus has changed to become less severe, explaining low hospitalisations & deaths.

Instead, the increase in cases in younger people is likely responsible for this. The slower rise overall is thanks to masks, work from home, etc.

In a way we are 'lucky' because the rise in cases in younger people is sounding an alarm without the tragedy of high death counts. Our containment isn't working: transmissions are rising.

This *should* be a chance to avoid that outcome! Act now, & get cases under control.

Unfortunately in many countries this isn't what's happening. Lack of deaths = lack of action. We seem to be 'waiting-&-seeing' - just like Florida in June.

And... I get why countries want to do this. We are all so, so sick of #COVID19.

The idea of further economic damage is horrifying. The fear of political unpopularity for taking action & then 'nothing happening' is real.

We would all prefer if we could ignore this, focus on recovery, and it will just 'be ok.'

But wishful thinking & denial won't help here, just like they didn't help many countries in March. 'Wait & see' won't work either. As we should have learned months ago, by the time you've 'seen' - it's much harder to get cases back under control.

And - we're heading into an big game-changer: winter. There's a very real risk that transmission will get much worse as the seasons change. @richardneher sums it up wonderfully:

A second big spike in cases will cause more economic damage, require reimposing restrictions, hurt population's health, & interfere with our lives. We want to avoid this as much as we can!

So we should head into winter with as few cases as possible - & a good plan

We had Italy in Feb & Florida in June...

The curse of #SARSCoV2 seems to be that we aren't willing to look at what happens to others & imagine it could happen here too.
We want to wait & see it ourselves.

We can break that curse - let's act now.

Why worry so much about what age groups are getting infected? Because age greatly influences #COVID19 outcome!
@richardneher shows predicting case fatality ratio(CFR) on changing infected age groups matches what we observe. More cases in young = lower CFR
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