COVID-19 and H1N1
When faced with the ever mounting death toll of COVID-19, it can be rather easy to feel like the world might be ending. I in no way mean to downplay the monumental tragedy this pandemic has been and continues to be, but as “anti-mask” and “anti-lockdown” sentiments rise, it is more important now than ever to understand just how bad things could have been—and may yet be if we cannot stay the course.
To this end, it is extremely relevant to investigate the meaning of R0 (read R nought) and what it tells us about the spread of an epidemic. It is a measure of a disease’s transmissibility at its inception, or time t=0. It is defined to be equal to p*k, where p is the probability that contact between an infected and non-infected person will result in transmission and k is the number of people an infected person will make contact with while contagious. Note that this does not have to be in person, since diseases can spread off of surfaces so sometimes “contact” can be something as small as touching the same doorknob. R0 is an extremely quick way to get an intuition for how quickly a disease will spread. For instance, if k=0 or p=0, the disease dies immediately since no one other than patient 0 ever gets it. Alternatively, if k=1 and p<1, the disease gets passed around from person to person until someone fails to transmit it, at which point it dies. However, as long as pk>1, the disease will continue to spread, since the expected outcome is that more than one person is infected at each step. To stop the spread, it is therefore imperative to lower this transmissibility. Lowering p involves things like surgical masks, vaccines, and sanitizing surfaces, while lowering k involves things like social distancing and closing restaurants. For a disease like COVID, much of what can be done to lower p is being done: Many places have mandatory masks, high sanitation standards, and vaccines are in the early stages of actual delivery at the time this is being written. The brunt of the work, however, is in reducing k. This is what all the “in this together” and “do your part” is about. Lowering k relies on everyone collectively taking the disease seriously and keeping contact to a minimum. This is also where it becomes most pertinent to compare COVID-19 to H1N1, the virus behind the Spanish flu.
Dubbed the Spanish flu due to Spain being the first place to accurately report the toll of the disease (leading people to believe it was worst in Spain and so must have originated there), H1N1 would eventually go on to infect over 500 million people (a third of the world’s population at the time). Estimates place the number of deaths anywhere between 17 million and 100 million, though the most consistent estimate is about 50 million. A major reason behind this is that when it began to spread in 1918, conditions seemed almost custom-made to maximize k. World War 1 was nearing its end. Soldiers were stuck together in cramped trenches, often literally huddling together for warmth as weather grew colder. Men on the front lines were the first victims of H1N1, and since this was an international war, the worst possible thing to do in a pandemic was happening: Soldiers were being sent home. It is difficult to imagine how a disease could be spread more efficiently if world governments were actively trying to do so.
In November, the war ended, and though hundreds of thousands of people had already died in America alone, the combination of returning soldiers and rampant celebration further strengthened the spread of the virus. People had already grown weary of the epidemic, willing to engage in a little risky celebration to feel like the world might not be ending, and for many, it killed them. This next statement is meant not to instill fear, but rather to highlight how successful countermeasures can be and how important it is to continue employing them, no matter how weary we may be of wearing masks and staying indoors. For the Spanish flu, estimates place R0 at about 2.0. For COVID-19, the consensus is 2.5. COVID-19 has been deemed significantly more infectious than H1N1, and yet through constant vigilance, the death toll is barely one 50th.
Before the disease was eventually contained, it is estimated anywhere between 500,000 and 850,000 Americans died. At present, we have recently surpassed 300,000 COVID deaths. With a vaccine in sight, things are hopeful, but there is much work to still be done, and we cannot relax yet. This is perhaps best illustrated by the following adjacent events on Dr. Milsten’s timeline
- November 21st, 1918: “Sirens sound in San Francisco announcing that it is safe for everyone to remove their face masks.”
- December 1918: “5,000 new cases of influenza are reported in San Francisco.”
As stated in the article comparing COVID-19 and the Spanish flu, “eradicating SARS-CoV-2, as was done with SARS-CoV in 2003, does not seem a realistic goal in the short term”. As exhausting as it may be, we must continue to fight to keep k down as new ways to decrease p are developed. Despite mistakes and setbacks, our modern approach to the pandemic has already been wildly more successful than the containment of H1N1 in 1918. The fact that COVID-19 patients are contagious before they are symptomatic (and often not symptomatic at all) only pushes k higher, making our continued efforts to mask and social distance critical. Many people have hailed imminent vaccines as silver bullets, but R0 can only be kept low if we continue to work k down, even as a vaccine is disseminated. k alone has not been enough to stop the spread, and the drop in p from a vaccine won’t be either. It requires continued work on both fronts. We can’t rest yet. But we are so very close.
Comparing SARS-CoV-2 With SARS-CoV and Influenza Pandemics (pdf download)
1918 Influenza Pandemic: A United States Timeline