Recently I described how I monitor the disease indicators published by government agencies, and how I’ve set a threshold of 6 people hospitalized per hundred thousand residents for deciding when to relax out of “outbreak mode” and eat and sing indoors.  So then, when to go back into outbreak mode and start taking precautions again?

I need to stress again here that I am not a doctor or an epidemiologist.  I’m not qualified to make these decisions, and I wish we had guidance from experts.  Unfortunately, the experts are assuming that everyone cares only about themselves and provide no guidance for people who care whether they pass these diseases on or develop long term conditions.  I do care, so I’m figuring this out as best I can.

Hospitalizations and deaths are trailing indicators: after infection it can take weeks for people to become sick enough to go to the hospital, and longer to die.  That means we need to look at leading indicators like case counts and wastewater concentration.  I’ll talk about wastewater in a future post.

Case counts are less reliable than hospitalizations and deaths, because they depend on the number of tests administered.  As we know, the number of tests dropped precipitously when President Biden declared an end to the State of Emergency and stopped reimbursing for tests. Providers tend to administer tests when they think people might be sick, so if they don’t anticipate infections they may not find them.

Case counts could be inaccurately low, so we should always keep an eye on hospitalizations.  There’s a chance that case counts could be inaccurately high, but if that happens, the worst that could result is that we think an outbreak is more severe and take too many precautions.  Of course, it’s better to err on the side of caution.

So if we’re looking at case counts, what is a good threshold for going into outbreak mode?  This turned out to be a lot more complicated than I thought, but let’s start with the basics and assume that all the data is three weeks old.

I looked at the seven outbreaks we’ve had since the the first wave (when we didn’t have good tests) and found the point when the 7-day average of COVID hospitalizations in New York City went above 6 per lakh per day.  Then I looked at the case rates for the day three weeks earlier:

As you can see, there’s a fair range of variation.  But let’s pick a rate that’s in the more common range, say 60 cases per lakh population per day.  How much warning would that give us?

It looks like anywhere from 2-7 weeks, usually about three weeks, which is basically what we want.  If we see a case rate above 60, that means that the hospitalization rate is likely to go above 6, and may already be above 6.  That’s a sign that it’s time to go into Outbreak Mode.  For me, that means moving karaoke online, avoiding indoor dining and social events, and wearing a mask in all indoor public spaces.

Keep in mind that COVID is only one of many infectious respiratory diseases that can kill or disable.  I chose the rate of 6 hospitalizations per lakh per day because that was what we were prepared to tolerate during the 2018-2019 influenza season.  The hospitalization and death rates we care about are for all these diseases, but flu and RSV are not as well documented as COVID.  Hospitalization rates for flu and RSV are reported nationwide, but the local reports for New York only give absolute numbers for cases and hospitalizations for those diseases.

What I tend do do is to estimate a ratio based on the total case counts for all three diseases, or on the nationwide hospitalization rates.  If RESP-NET is reporting that there are as many flu and RSV hospitalizations as COVID hospitalizations nationwide, I’ll assume that that applies to New York.  If New York’s flu and RSV report shows that there are half as many positive tests for flu and RSV as for COVID, I’ll go into outbreak mode at 40 COVID cases per lakh instead of 60.

Stay tuned for that post about wastewater concentrations!