What Do The Models Say About What Happens Next?

We are now isolating ourselves, at least the non-MAGA portion of the population. The numbers of cases of Covid-19 – and deaths – are mounting. What can we expect? What happens next?

Lots of us are wondering what comes next in the SARS-CoV-2 pandemic. We already have an indication in New York City. At the same time, many states and countries are instituting lockdown measures that should decrease the numbers of sick and dead. I’ll use the Imperial College paper to discuss what is likely to come next.

A few words first about the Imperial College model, and models in general.

Neil Ferguson, one of the modeling team,  testified to Parliament that he was becoming “reasonably confident” that the UK would be able to handle the peak of the epidemic in two or three weeks, with a death toll unlikely to exceed 20,000. This is being spun among those who want to send the US back to work as an indication that the model is wrong. IT IS NOT. What Ferguson is saying is that the UK is now taking measures to change the worst-case modeling scenario. THAT’S THE PURPOSE OF MODELING – to work out possibilities and recommend a path forward.

The Imperial College paper modeled the spread of the virus uncontrolled, and then investigated the effect of social distancing measures. The conclusion was that all the measures studied were required to suppress the growth of COVID-19 cases and avoid overloading the hospital system. I summarized that part of it in my earlier post.

Because the SARS-CoV-2  virus is new to the human race, nobody had immunity to it when it was first introduced. It can spread with no barriers to every human on the planet. As people are infected and recover, they become immune to the virus and cannot become ill from it again. It’s not clear how long that immunity lasts, but if it is even for a few months, it will slow down the spread of the virus.

Social distancing measures also slow down the spread of the virus, but they don’t confer immunity, leaving enough of the population susceptible to the virus that, when the distancing measures are lifted, the virus can spread again. The more successful the distancing measures are, the fewer people will become immune, and thus, the larger the secondary peak. The secondary peak can be controlled by reimposing distancing measures. A target might be set, and a series of secondary peaks at levels that will not break the hospital system would result  (Figure 4 of the Imperial College paper).

Imperial College 200329

As a modeling projection goes further into the future, its results become less reliable. This is partly because of uncertainties in the parameters, but it is also because we are taking measures to control the spread of the disease. Distancing helps. If we can make testing general for both the virus itself and for immunity to it, that will change things significantly.

There are several things we don’t know that make a difference in how things will play out. We have some idea about them, but as we learn more, the model predictions may change.

How long do people spread the virus?

How many people are infected by the virus and never show any symptoms but become immune? This is important because the more people become immune this way, the lower the peaks in the model and the faster they die out. When enough people are immune, the virus cannot propagate. That’s herd immunity. We don’t know what percent of the population must be immune to establish herd immunity.

What kind of immunity do people develop after having the virus? How long does it last. Is it a strong immunity, or do some people remain susceptible to reinfection?

Does the virus becomes less infective during the summer? If it does, infections could decrease but peak again in the fall and winter. Or the virus could weaken to cause a less dangerous disease. That’s not unknown in pandemics.

The single biggest factor in filling in what we don’t know now is testing. Test people with symptoms. Test people without symptoms. Test adults. Test children. Test people near the people with symptoms. Test medical workers, them especially. Tracking cases to see who should be quarantined.

A new test promises results within five minutes. That is a test for the virus itself. We also need serology tests, to determine if people are immune to the firus.

Testing will give individuals and their medical support more information about their next actions. It will also provide information about the course of the disease and information that can be used in modeling.

We are buying time with the measures we are taking now. We can’t shut down the economy for long, and Trump and his admirers are already pressing to open things up again. Some of them are ignoring distancing now.

The answer, eventually, will be a vaccine. Several groups are working on vaccines, and one clinical trial is in progress. Eighteen  months is commonly cited as the time period, but it could take longer. We don’t know.

Cross-posted to Balloon Juice


  1. Timothy Wozniak · April 5, 2020

    Thank you for taking the time to research and write about models. Interesting how IHME and a CHIME differ. Same with Imperial. In my Indiana there is a county difference in adherence to statewide social distancing mandate. As you note, testing (throat and serum are the key) to refining models. As a retired chemist, I enjoy your blog.


    • Cheryl Rofer · April 8, 2020

      Thanks! I hope to write a post on CHIME next.


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