Category: Projections

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Planning Projections

What’s Next?

Our situation is improving. Recent updates to the modeling that most health and government officials are using to gauge the pandemic suggest that we are flattening the curve. Growth of new cases and hospital admissions appear to be coming down, according to the respective Governors of New York and New Jersey. There is a light at the end of the tunnel. Whether it’s daylight or another COVID-19 freight train could depend on our decisions and actions over the next few weeks to months. What comes next may be up to us.

If we are at peak, close to the peak, or perhaps even past the peak, what can we expect the COVID-19 situation to look like over the next few weeks and months? When can we start getting back to normal (or at least our new normal)? The answer is not entirely clear, but the models are giving us an idea. And a choice.

If we relax social distancing all at once…

We could see a resurgence of COVID-19 infections, perhaps close in magnitude to that of the first wave of cases. Why? Because, as far as we know, only a fraction of the population has been exposed to the virus and has immunity. Some recent data suggest that, on average, one individual with COVID-19 infects between five and six other people. This means that as more than 80% of the entire population would need immunity to COVID-19 to actually stop the spread (via herd immunity). In other words, 8 out of 10 people would need to become infected (or vaccinated, but a vaccine is probably more than a year away).

The point here is that we cant simply “reopen the US for business” all at once, or else we risk ending up back where we started. Though we have certainly learned a lot about COVID-19 and our hospitals will likely be better prepared, we do not want to risk flooding the healthcare system and putting more lives at risk.

Even if we extend the stay-at-home directive for another 30 days, but then completely remove it along with the other social distancing measures, we risk another wave of cases – a second peak – just the same.

If we relax social distancing over time…

We could settle into a much lower plateau of new infections – a steady flow of new COVID-19 cases spread out over a longer time period. If it is true that over 80% of everyone would need to become infected, then we would want that process to take as long as possible, with as few peaks (rapid influxes of new cases) as possible.

A projection from the Federal COVID-19 Data & Analytics Task Force suggests that if we maintain a “steady state” of partially relaxed social distancing, we could achieve this steady state of infections. This involves keeping schools closed through the school year (through summer), and at least 25% of employees still telecommuting, along with other social distancing measures, perhaps like limiting the number patrons at restaurants and the size of public gatherings.

This leads to arguably the most important question we need answered.

Who can safely leave their home?

One of the reasons why we don’t know when we will be able to return to normal is because we simply have no idea who and how many of us have been infected by and developed immunity to COVID-19. The game-changer in determining how we, as a country, will recover from this pandemic will be the availability of antibody testing.

After someone has been infected with the coronavirus and recovered, their body has antibodies to the virus that can be identified in a test. If someone tests positive for coronavirus antibodies, they are presumably immune to the virus. (I say presumably because we are not yet entirely sure about risk of re-infection, particularly for patients who had very mild cases the first time around).

As we begin to relax social distancing and return back to normal life, those who have immunity to COVID-19 would be able to safely leave their home without risk of contracting or spreading coronavirus. Unless you were tested, confirmed to have COVID-19, and have since recovered (per CDC guidelines), you have no idea whether or not you’ve had the virus. Remember, some estimates say up to 50% of cases have no symptoms at all whatsoever. It is possible, if not probably, that for every case that has been officially diagnosed, there are 10 more cases that were never diagnosed and resolved without any medical attention whatsoever.

With this in mind, perhaps younger, generally healthy people could return to work even without immunity because of the lower risk of severe symptoms. The problem is that we don’t know for sure. Early data show that younger Americans with COVID-19 are still being hospitalized. Also a major problem is that asymptomatic carriers could spread the virus to older individuals or others with underlying health conditions. So until we have access to an antibody test, we are essentially blindly making decisions about when and how to relax social distancing.

Not out of the woods

And we may not be for a while.

Social distancing is having a major impact on slowing the spread. However, this does not mean that COVID-19 has stopped spreading. Nor will it stop spreading just because we pass the peak. Case numbers continue to rise. Lives are still being lost.

Some of us will be home through May. Possibly into June. Possibly again come fall. There is some evidence that there could be a seasonal resurgence of COVID-19 when the weather cools.

While we may be able to begin relaxing social distancing measures and begin returning to some semblance of normal life as early as next month, those decisions and policies will likely be state and city specific. Population density does influence the spread of the virus. Metropolitan areas will likely need to very slowly relax the stay-at-home and social distancing directives over a longer period of time.

A new normal

Face masks? No handshakes? No fans at sporting events? Will we think twice about going to dinner and a movie? One thing we can be sure of is that, regardless of when we start to return to the outside world, our world will look a lot different for a long time, if not forever.

So where do we go from here? For now, we keep doing what we are doing. We stay home. But when we start to return to normal – our new normal – we need to do it cautiously and wisely. Ideally we will make those critical decisions based on evidence and testing, and not just based on days on a calendar.

UPDATE: 4/11/20 10:30AM – New York City has announced that public schools will be closed for the remainder of the school year.

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Projections

Peaking Too Soon

My apologies for constantly bombarding you with numbers, and models, and projections (oh, my). But 1) I am an epidemiologist and it is my job, and 2) major media outlets are running with the story of a new COVID-19 projection that deserves your attention.

Released alongside a preprint publication yesterday, the Institute for Health Metrics and Evaluation at the University of Washington has developed a data visualization tool that projects national and state-by-state COVID-19 curves. By utilizing case, hospitalization, and death rate data from across the US, the tool projects when the peak of the curve will occur. Even with the social distancing measures of all 50 states factored into the model, it predicts that the US will reach the peak of hospital utilization and the peak number of daily COVID-19 deaths within two weeks, on April 15th.

US Projected COVID-19 Peak: April 15th

Based on the model at the peak of hospital resource use, the US as a whole may be short more than 50,000 hospital beds including a shortfall of more than 13,000 ICU beds.

The number of COVID-19 deaths per day in the US is estimated to reach 2,200 (with a possible range of 1,100 to 3,300) at peak on April 15th.

In total, the model predicts just over 81,000 COVID-19 deaths in the US by June 1st, with a possible range anywhere from 36,500 to 144,500 deaths.

To me, these models are interesting but they are still only models at the end of the day. They are projections of what may occur, but they are not necessarily predictive, and certainly not prescriptive, of what will occur in the US.

What I find more informative about these models are some of the state specific projections – not so much the numbers but rather the timing. Take my state, New Jersey, for example. It would seem we are ahead of the national curve, and not in a good way.

NJ Projected COVID-19 Peak: April 8th

The model predicts the peak of hospital resource use to occur in approximately one week, on April 8th.

Statewide, we will be very close to capacity in terms of overall beds, and almost at double the capacity of ICU beds.

The number of deaths per day peaks by April 9th at 90, with a range of 50-125 deaths per day.

By around May 1st, New Jersey total deaths hits its plateau (a full month ahead of the US as a whole).

Statewide, total deaths are projected to be between 1000-2600, with 1850 being the point estimate.

It would appear that New Jersey (and neighboring New York) hit peak a week earlier than the nation reaches its peak. It is certainly possible, though health and government officials around here have certainly been hoping to push the peak out at least a few more weeks longer than the seven days this model is giving us.

Just as states across the US have different curves and timing, it is probably that cities and communities within states also have different curves. These projections are based on state-wide data and are not specific to any one part of the state. Our city implemented social distancing measures about one week before there were state-wide mandates. Yet, our city is more densely populated and closer to the NYC epicenter than other parts of the state.

I wish to reiterate that these mathematical models are imperfect. They are based on current evidence, which is limited, and are not necessarily indicative of the actual situation. We already know that we are under-identifying and underestimating total cases. However, we should still do our best to prepare for a sooner-than-anticipated peak.

It is also unlikely that COVID-19 spread will simply level-off around May 1st in New Jersey or June 1st nationwide. As we begin to relax social distancing measures, the spread of COVID-19 can be expected to continue, but hopefully at a much lesser rate. In addition, a possible seasonal effect on COVID-19 transmission could bring us a resurgence in the winter months, especially if all social distancing measures have been removed.

Explore the Institute for Health Metrics COVID-19 Projection Tool
COVID-19 Projections

Note that in these visualizations, the dotted curves represent the single point estimates. The large swath of color surrounding those lines depicts the wide range of possibilities surrounding that estimate.

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Planning Projections

The Case for Social Distancing

As hospitals scramble to prepare, they are asking the question, just how bad could it get? As I was trying to find answers to help inform our local hospitals, I struck gold with the COVID-19 Hospital Impact Model for Epidemics (CHIME), an online tool developed by the Predictive Healthcare team at UPenn Medicine.

The tool helps project the expected number of daily hospital admissions, intensive care unit (ICU) admissions, and admitted patients in need of a ventilator, as well as admitted patient totals, based on a number of assumptions about the community, hospital system, and spread of the virus. It is not predictive, but rather a tool that could be helpful in planning for the anticipated demand on the local hospital system.

I ran an analysis for Small City, USA. My model assumptions were as follows:

Model Assumptions

1 known case
1 known hospitalization
6 day case doubling time
5% case hospitalization rate
2% case ICU rate
1% case ventilated rate
7 day hospital length of stay (LOS)
9 day ICU LOS
10 day ventilator LOS
50% hospital market share (we have two main hospitals)
65,000 population


The key consideration here is case doubling time. A 6-day case doubling time simulates the spread of COVID-19 unchecked, without any mitigation efforts like social distancing. In other words, we are modeling the worst case scenario, given what we know and where we are today. Here are the results.

Figure 1: Projected number of daily COVID-19 admissions

Without mitigation, COVID-19 hospital admissions peak at 56 days (8 weeks) with 18 new admissions per day, including 6 ICU patients, and 3 on ventilators (Figure 1). Because average length of stay is 7 days for each admitted patient and longer for those needing intensive care, the projected total number of admissions on day 56 paints a dire picture.

Figure 2: Projected total COVID-19 patients, accounting for arrivals and discharges

At its peak, COVID-19 patients would account for 122 hospital beds in 8-weeks time, with 62 patients in the intensive care unit, and 34 on ventilators. It is unclear whether our hospitals currently have, or will have, the beds, ventilators, supplies, and personnel needed to treat this number of patients (Figure 2). And what does the rest of Small City look like?

Figure 3: The number of infected and recovered individuals in the hospital catchment region at any given moment

At around 9 weeks, without social distancing measures, we would see more than 17,000 cases at the peak of the COVID-19 outbreak. That is nearly 1 in 4 Small City residents (Figure 3).

Social Distancing is the Difference Maker

Now back to that oh-so important case doubling time. If we can add just 2 days to the amount of time it takes for the number of cases to double, from 6 to 8 days, best achieved though social distancing measures, we can push the peak out by 2 weeks and reduce the number of daily hospital admissions by more than 40% (Figure 4).

And the more social distancing the better. If we can reduce social contact by 25%, we see a 65% drop in daily admissions (Figure 5), and the total number of community cases drops from 17,000 to just over 10,000 at peak (Figure 6).

There is a reason why schools are closed, businesses are closed, and we are being asked told to isolate at home. Social distancing works. Without it, we risk overloading our hospitals and putting COVID-19 patients – and healthcare workers – in a very dangerous place.

For more on how social distancing works to slow the spread of COVID-19, check out this Washington Post animation.

Figure 4: Projected number of daily COVID-19 admissions, with CDT of 8 days
Figure 5: Projected number of daily COVID-19 admissions, at 25% social distancing
Figure 6: The number of infected and recovered individuals in the hospital catchment region at any given moment, at 25% social distancing
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