Thursday, March 12, 2020

COVID-19 compared to other diseases

Updated 14 March 2020 for clarity.

Since my last article, a lot has happened. COVID-19 has been declared a pandemic by the WHO. Italy has quarantined the entire country. Ireland has shut down all schools and day-care, banning large public events, including the upcoming St. Patrick's Day parades. These are reactions to the medical reality of the current disease.

I am seeing people on social media comparing COVID-19 to the catastrophic 1918 Spanish influenza. On the other extreme, some say that this pandemic is just another flu. Who is right? This article will use medical sources to answer this important question.

Unlike last time, I won't annotate each fact, but all important information does indeed derive from the references provided.


1918 Spanish Flu

The 1918 influenza A/H1N1 outbreak was deadly, infecting one-third of the world's population (500 million) with fatality rates greater than 2.5%. There were 50 to 100 million deaths. This pandemic is known colloquially as the Spanish flu, largely because Spain reported on the disease early, while other governments were still engaged in a cover-up.

While most flu (which is to say, influenza) pandemics produce the greatest mortality in the young and the old, the 1918 disease was unusual in that it also targetted adults 20–40 years of age. The reasons for this are still unknown, but the impact on society was dramatic.

It is important to understand that once a virus exists, it sticks around. Almost all subsequent cases of influenza A (including the strains H1N1, H2N2, H3N2) have been caused by descendants of that 1918 virus.


Disease analysis (epidemiology)

Those who analyse infectious disease have many medical and statistical tools at their disposal. The goal is to accurately characterise a viral outbreak, so that lessons can be learned to save lives in the future.

There are several factors that describe the potency of an infectious disease. But be aware that these are descriptive terms, used after the fact to help compare diseases. They do not describe innate characteristics of the virus itself.

The reproduction number R is the average number of secondary cases generated per infectious case. It helps describe how quickly the disease will spread through a population. If the value is less than 1, an outbreak is highly unlikely. If R is 2, each infected person will infect, on average, two others. It's easy to see how this scenario would allow a disease to spread rapidly. In fact, it's the classic exponential growth curve.

Some diseases affect only those individuals with particular genetic or other factors. But in the case of influenza and coronavirus, we're all susceptible. R is then known as the basic reproduction number R0 (pronounced "R naught"). For our purposes R and R0 are the same.

There are other complex factors that influence how diseases spread, including the homogeneity of a population and the inherent herd immunity. SARS-CoV-2 is a new virus so the human body has not had the time to build up specific defences. We have no immunity.

The R factor can nonetheless indicate which countermeasures are appropriate to slow transmission of the disease. These measures include social distancing, treatment, and vaccination.

With an R number less than 1.5, the spread of the virus can be controlled with less than 50% of contacts successfully traced. However once R gets to 2.5, over 70% of contacts must be traced to control the spread. For R greater than 3.5, more than 90% of contacts must be traced for successful containment. This explains why China deployed over 8000 epidemiologists to Wuhan, tracing tens of thousands of contacts a day. The R factor was known to be relatively high, so they were not taking any risks.

The other factor of great popular interest is the mortality rate. This is more correctly termed the Case Fatality Ratio (CFR), which indicates how many people die out of the total number of infected. That number in the denominator is a matter of great conjecture for SARS-CoV-2. When the number of infected are under-reported, we might calculate a CFR that is too high, resulting in unnecessary alarm.

A good number of overly-positive articles are now being published that make this assumption. But to simply assume that the CFR is too high is bad logic. The CFR is what it is, a descriptive indicator of our best knowledge at present.

The CFR number relies heavily on correct identification and treatment of victims. According to the WHO, the CFR was 17.3% for Chinese cases with symptoms that began 10 January 2020. But the CFR reduced to 0.7% for patients after 1 February, once the disease was identified and stringent procedures (social and medical) were put into place.

Again, CFR is descriptive, allowing us to put a number on what happened. It is not a good predictor of the future. Except that it can encourage us to do everything possible to fight the new disease.


Disease comparison

I have compiled a comparison table from available sources. R and CFR are estimated. The high CFR for Ebola and AIDS are true only when the disease is untreated. pa = per annum.



What we can conclude from this is that COVID-19 has an R factor higher than seasonal flu and very likely higher than any influenza. At the same time, the CFR is much higher than any previous influenza except perhaps the 1918 pandemic.

COVID-19 is not business as usual. It is not just another flu. In fact, it is not a flu at all, but more closely related to the common cold. Regardless, it is already much deadlier than SARS or MERS.

However, we are much better prepared for a pandemic than the world was in 1918. The age-related characteristics of mortality do not have the unique (and devastating) properties of the Spanish flu. There is no indication that this virus will damage society on that scale.

However, vigorous contact tracing is required to find existing cases. Isolation is then paramount. The measures taken in Italy and Ireland are not over-reactions but necessary steps. Indeed, all of the medical facts have been known for some time. Governments could have reacted two to four weeks ago. But they waited until the economic pressures were such that further delay would be perceived negatively.

The best thing you can do now is assume you have been infected. Carry out all the recommended practices and self-isolate. Watch for symptoms. Enjoy time with your family. Support medical practitioners wherever possible. Do not hoard food or panic-buy. For one thing, this exposes you to other people at a time when this is a bad idea.

In the future, consider the benefits of a more robust health care system, and a basic income that would protect all workers in times of crisis. Think more carefully about who you elect, considering their policies on this issue.

A society is not measured by wealth accumulation, but by how the weakest and most vulnerable members of society are treated. SARS-CoV-2 has arrived to deliver this message. It won't be the last virus mutation.


References

Biggerstaff, Matthew; Simon Cauchemez; Carrie Reed; Manoj Gambhir; Lyn Finelli. 2014. "Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature." BMC Infectious Disease 14, 480. Available: https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471-2334-14-480

Fine, Paul E. M. 1993. Herd immunity: history, theory, practice. Epidemiology Reviews 15.2, 265-302. Available: https://academic.oup.com/epirev/article/15/2/265/440430

Hellewell, Joel; Sam Abbott; Amy Gimma et al. 2020. "Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts." The Lancet, 28 Feb 2020. Available: https://www.thelancet.com/journals/langlo/article/PIIS2214-109X(20)30074-7/fulltext

Taubenberger Jeffery K., David M. Morens. 2006. "1918 influenza: the mother of all pandemics." Emerging Infectious Disease 12.1, 15-22. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3291398/

Worldometer. 2020. "Coronavirus (COVID-19) mortality rate." Worldometer [website], 5 March 2020. Available: https://www.worldometers.info/coronavirus/coronavirus-death-rate/

World Health Organization (WHO). 2018. "Influenza (Seasonal)." World Health Organization [website]. Available: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)

World Health Organization (WHO). 2020. "Report of the WHO - China Joint Mission on Coronavirus Disease 2019." World Health Organization [website]. Available: https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf

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1 comment:

robin said...

This article has some handy graphics:

https://www.vox.com/future-perfect/2020/3/12/21172040/coronavirus-covid-19-virus-charts

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