COVID-19: What to Expect

March 12 update: the situation has deteriorated rapidly. See our latest writing.

This is part of a series about the COVID-19 (coronavirus) pandemic. See also:
COVID-19: General Advice
COVID-19: Specific Advice

This post was written by Tornus (who has no professional qualifications but has an extensive amateur interest in epidemiology) and Dr Smartypants, who is a physician and infectious disease epidemiologist with extensive experience working at the CDC and public health departments. We have endeavored to provide accurate, concise, and actionable information here. However, the best source of information about COVID is the official CDC page. You should trust the CDC more than us (or anyone else).

Summary

Over the last few days, we have learned enough to say with confidence that COVID-19 is a serious global pandemic and now is the time to take significant precautionary measures.

There is a great deal that we don’t know yet. It is quite possible that COVID-19 will be no worse than the seasonal flu and it is quite possible that COVID-19 will be as bad as the 1918 flu pandemic. We expect to have much more clarity about the situation by mid to late March.

How bad will it be?

Right now, there’s a fairly wide range of plausible scenarios. We know COVID-19 is not going to fizzle out like MERS or SARS did, and we know it isn’t going to end civilization. This is serious, but it isn’t the zombie apocalypse. What happens over the next year will probably fall somewhere between two scenarios:

Best likely scenario: a typical flu year

The best likely outcome is that COVID-19 is like a typical flu season:

  • 10% of the world gets infected
  • 0.05% of infected people die
  • Globally, about 400,000 people die
  • Most deaths occur among people over 65 and/or especially susceptible to respiratory infections
  • Social and economic impacts are fairly minor

Worst likely scenario: the 1918 flu pandemic

The worst likely scenario is that COVID-19 is roughly comparable to the 1918 flu pandemic:

  • 50% of the world gets infected
  • 2% of infected people die
  • Globally, about 80,000,000 people die
  • Most deaths occur among people over 40 and/or especially susceptible to respiratory infections
  • Children and young adults are likely to be much less severely affected
  • Substantial social impacts including extensive voluntary and government-mandated closures
  • Substantial economic impacts including a global recession
  • Catastrophic impact on travel, hospitality, and food service sectors
  • The acute phase of the pandemic lasts for 3 - 18 months
  • COVID-19 recurs annually, just like the flu and the common cold

We want to put the worst-case scenario in perspective. You should take this very seriously: it is entirely plausible that more people will die in 2020 than have ever died in any single year, and several people you know will die. But even in the worst case, it is likely that you and your immediate family will experience nothing worse than a bad cold. Governments will function normally, the lights will stay on, and your supermarket will remain fully stocked with food. Take this seriously, but do not panic: this is not the zombie apocalypse.

Now is the time to act

We currently have limited data about COVID-19 from China and a few other countries. Over the next few weeks, will will get data that is much higher quality, much more extensive, and much more relevant to predicting the course of the pandemic in North America and Europe. By mid to late March, we may learn that COVID-19 is less severe than we had feared and does not require extensive precautionary measures. However, we may also learn that this is going to be a very severe pandemic that requires extensive and lasting precautions.

We know from the 1918 flu that acting quickly is critically important to reducing the severity of a pandemic: a delay of as little as two weeks can substantially increase the final death rate. Therefore, given that we are now experiencing community transmission in North America, we believe it is prudent to take immediate precautionary measures.

If we’re lucky, we will be able to relax those precautions in a few week. And if we aren’t lucky, acting early will have saved lives.

Nerdy details: what do we know?

Understanding the spread of new diseases is incredibly complicated, and we’re going to greatly simplify some very complex topics. But for the nerds among you, here’s some of the data that informs our thinking. You can skip the rest of this post unless you want to nerd out with us.

When judging the impact of a disease, we want to know two things in particular: how easily it spreads and how deadly it is.

Epidemiologists often refer to the R (reproduction) value of a disease. R is how many people each infected person spreads the disease to. So a disease with an R < 1 will tend to die out, while a disease with an R > 1 will tend to spread to an increasing number of people. Many factors affect R, including how many people have already been infected and what precautions are in place to prevent spreading.

A related question involves asymptomatic spreading. One reason MERS and SARS didn’t spread widely is that they very quickly caused severe symptoms, so sick people were easy to identify and tended not to circulate in the community.

Severity is a complicated topic, but the simplest measure of severity is case mortality: if 100 people catch the disease, how many will die? Case mortality typically varies with age: older individuals are usually but not always impacted much more severely than young and healthy people.

Comparison data: seasonal flu and the 1918 flu pandemic

Two good points of reference are the seasonal flu and the 1918 flu pandemic.

Flu varies from year to year, but in a typical year, seasonal influenza:

  • Has an R of about 1.3
  • Infects about 10% of the global population
  • Has a case mortality of about 0.05%
  • Kills about 400,000 people globally

Our best guess is that the 1918 flu pandemic:

  • Had an R of about 1.8
  • Infected about 30% of the global population
  • Had a case mortality > 2.5% (likely much higher in some areas)
  • Killed about 50 million people globally

What do we know about COVID-19?

Right now, all of our data about COVID-19 is extremely limited: expect substantial changes as we learn more.

What we know so far about infectivity isn’t encouraging. Current estimates of R range from 2.2 to 2.6, considerably higher than even the 1918 flu pandemic. In addition, there is some evidence that during the first week of infection many people are contagious but experience only mild symptoms. This complicates containment efforts and means that contagious individuals are likely to circulate in the community.

Case mortality estimates currently range from 1.4% to 3%.

What don’t we know?

There is a great deal of uncertainty about pretty much everything. Some of the limitations of our current data include:

  • The full course of the disease seems to last for several weeks, with symptoms and mortality getting worse over time. Much of our current data is based on cases that are still in progress, so we don’t know their outcomes.
  • Surveillance has been very limited: until recently we didn’t know what to look for, and we still have very limited testing capacity. Consequently, we have no idea how many mild cases of the disease have gone undetected.
  • Because serious containment measures are quite recent, we have limited data on how effective they are.
  • Many respiratory diseases are more prevalent in winter. We don’t know if COVID-19 will follow this pattern.
  • Most of the current data comes from China, which differs from North America and Europe in many ways including population density, prevalence of smoking, and feasibility of strict government-mandated containment and social distancing.

There is some reason for optimism:

  • So far, the disease seems to be very mild among children and young adults.
  • It is quite likely that many mild cases have gone undetected, meaning that the actual case mortality may be much lower than has been reported (one recent paper suggests that true case mortality may be well below 1%).
  • Mortality may be lower in populations with fewer smokers.
  • Containment measures may significantly slow the spread of the disease.

There are also some reasons for concern:

  • Early data from Europe do not show a drop in case mortality. Italy has reported 1,577 cases with 34 deaths (case mortality of 2.2%).
  • Community transmission is now occurring in multiple countries including the United States.
  • The 1918 flu occurred in three waves, the second and third of which were much deadlier than the first. We cannot rule out the possibility that COVID-19 will follow a similar pattern.
  • Coronaviruses typically do not produce long-term immunity in infected individuals, meaning that COVID-19 is likely to become a recurring problem.

Further reading

Coronavirus Disease 2019 (COVID-19)

https://www.cdc.gov/coronavirus/2019-ncov/index.html
The CDC’s excellent information hub for COVID-19.

Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6815659/
A good summary of typical seasonal flu patterns. Calculates an annual average of 389,000 deaths from flu, 2/3 of them among people 65 and older.

1918 Influenza: the Mother of All Pandemics

https://wwwnc.cdc.gov/eid/article/12/1/05-0979_article
Excellent in-depth discussion of the 1918 flu pandemic. Cites a 33% global infection rate, with 500 million people infected and 50 million deaths.

Wikipedia: Spanish flu

https://en.wikipedia.org/wiki/Spanish_flu
General discussion of the 1918 flu pandemic (including a discussion of the reasons why it was inaccurately called the Spanish Flu). Cites a 27% global infection rate, with 500 million people infected and a death toll between 40 and 50 million. Note that Wikipedia cites two waves of flu, although the CDC identifies three distinct waves.

COVID-19: Navigating the uncharted

https://www.nejm.org/doi/full/10.1056/NEJMe2002387
February 28 article discussing recent data about COVID-19. Cites case mortality rates of 1.4% - 2.0% and speculates about the true rate being considerably less than 1%. Cites an R0 of 2.2.

Public health interventions and epidemic intensity during the 1918 influenza pandemic

https://www.pnas.org/content/104/18/7582
A detailed look at efforts to contain the 1918 flu pandemic. Cites a CEPID (cumulative excess P&I death rate) of 719/100,000 in Philadelphia (which waited 16 days after the first case to implement strong containment measures) versus 314/100,000 in St Louis (which waited 2 days).

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