For a good reason, people have turned to the previous experience of influenza pandemics to try to make sense of COVID-19.
Influenza and coronavirus have a lot in common regarding how spread by respiratory droplets and the surfaces they land.
The respiratory failure of COVID-19 patients echoed in descriptions of H1N1 influenza patients from 1918-19. Lessons learned from the 1918-19 flu pandemic’s nonpharmaceutical measures, such as physical separation and school closures, have justifiably driven this pandemic’s policies supporting nonpharmaceutical interventions as physical separation and school closures.
Waves of the disease characterized the drastic mortality of H1N1 influenza in three major peaks in 1918-19 and frequently mentioned in current discussions about reducing social distancing measures and opening up the world. People are nervously eyeing the “second wave” of influenza that arrived in autumn 1918, the pandemic’s deadliest time, as COVID-19 rates begin to stabilize in some parts of the United States.
During the pandemic, there were three waves of death: weekly combined influenza and pneumonia mortality, weekly combined influenza and pneumonia mortality, and weekly combined influenza and pneumonia mortality (the UK, 1918-1919).
The waves were similar all over the world. 1918 Influenza: The Mother of All Pandemics (Taubenberger JK, Morens DM. CC BY 4.0) Emerg Infect Dis. 2006;12(1):15-22.
Waves, on the other hand, invoke predictability, and COVID-19 has proven difficult to predict. Despite the essential lessons learned from past influenza outbreaks, the 1918 pandemic is not a blueprint for what could happen in the months ahead with COVID-19.
As historians and virologists, we agree that comparing two pandemics has led to a public misunderstanding about what to expect from “flattening the curve.” There are significant differences between the sociopolitical contexts of 1918-19 and those of today. The virus that causes COVID-19, in addition to apparent virologic variations between influenza and SARS-CoV-2, suggests that their courses are not perfectly balanced.
Influenza pandemic a product of that time
Citizens today will perceive the world in 2020 to be significantly more linked than in the past. However, World War I and the mobilization of soldiers provided an ideal environment for influenza to spread. Although the source of the deadly 1918 H1N1 strain is unknown, evidence suggests that soldiers on the move drove its spread.
The H1N1 flu was brought back to the United States by soldiers returning from World War I. (CC BY 4.0, Keystone View Company/Library of Congress Prints and Photographs Division)
Young men from the United States left their homes – remote farms, small towns, and crowded cities – to explore the globe. Thousands assembled in military training camps and on troopships before heading to the front lines in Europe.
Civilians continued to operate in critical areas of economic activity worldwide, which necessitated movement through the same transportation hubs as soldiers did. During these movements, the disease’s first wave hit in the spring and early summer of 1918.
Soldiers mingled with their global compatriots in war zones throughout Europe, Africa, and Western Asia. As they demobilized, they traveled back to their homes worldwide through significant transit hubs, connecting with more people.
In autumn 1918, a particularly lethal second wave of influenza spread linearly along rail and sea lines, then radiated outward to wreak havoc on previously uninfected populations worldwide. This time follows a less deadly third winter wave of disease in early 1919 in some regions.
Medical historians estimate that influenza killed 50 million people worldwide between 1918 and 1920, with 675,000 in the United States. Following that, the flu strain began to fade, most likely due to changes in the virus itself. The reality that the vast majority of people had already been exposed and either gained immunity or died.
Since the pandemic flu waves did eventually fade, it’s easy to imagine today’s pandemic taking a similar route. However, because of fundamental variations in the biology of SARS-CoV-2 and influenza viruses, it is challenging to predict COVID-19’s future based on what happened in the early twentieth century.
SARS-CoV-2 and flu are biologically different.
Both the new coronavirus and influenza have RNA as their genetic material. When they replicate, RNA viruses accumulate many mutations because they rarely double-check copied genes to correct errors during replication. These mutations may often result in drastic changes, such as the virus changing the species it infects or the cell receptor it uses, or being more or less lethal or spreading more or less rapidly.
The genetic material of influenza organizes in segmented chunks, which is unique. Because of this peculiarity, the virus will exchange entire RNA segments with other influenza viruses, allowing for rapid evolution. Influenza has a distinct seasonality, with outbreaks occurring more often during the winter months. Virus strains mutate as they circulate, oscillating seasonally between the Northern and Southern Hemispheres’ winter seasons. This ability to adapt quickly requires a new flu vaccine every year to protect against new strains that have arisen in the region since last year.
The lessons learned from the previous pandemic can apply to the present one, such as the importance of wearing masks to prevent the virus from spreading.
Coronaviruses do proofread their copied RNA to fix inadvertent errors during replication, which decreases their relative mutation rate.
From the first SARS-CoV-2 sequenced in December 2019 in Wuhan, China, to recently banked sequences from the United States, there are less than ten mutations in 30,000 possible locations in its genome virus, which has spread across the world and multiple generations of human hosts. Independent of entire genome section swaps, influenza makes 6.5 times more errors per replication cycle.
The relative genetic stability of SARS-CoV-2 means that natural variations in virulence due to mutation are unlikely to drive future disease peaks. The mutation is unlikely to lead to COVID-19 “waves” that are predictable.
It’s also unclear if SARS-CoV-2, like influenza, would be affected by the seasons. It has already developed itself in a variety of climates. It’s difficult to blame recent decreases in the incidence of new cases in hot weather since they’ve coincided with a slew of strict nonpharmaceutical treatments.
All of this suggests that COVID-19 case oscillations are unlikely to be as predictable as discussions of influenza “waves” in 1918-19 would indicate. Instead, as SARS-CoV-2 circulates in nonimmune communities worldwide, physical separation and mask use can help slow its spread and, hopefully, keep infection and death rates stable.
As states relax nonpharmaceutical treatments, the US is likely to experience a long period of steady new infections punctuated by local flare-ups. These outbreaks would drive by nonimmune people being exposed to the virus, not by SARS-CoV-2 mutation or virulence. What people do in the future would almost certainly trigger an increase in COVID-19 cases and deaths.
This situation will continue until the United States’ population develops herd immunity, which is best achieved by vaccination. Regrettably, this phase can take years rather than months to complete.
One virus’s pattern is not a prediction.
People are looking for answers from influenza outbreaks in 1918-19 for a straightforward reason: it ended.
Without the advantage of an influenza vaccine (available only in the mid-1940s), a molecular or serologic examination, successful antiviral treatment, or even the assistance of mechanical ventilation, the pandemic ebbed after a final, third wave in spring 1919.
If SARS-CoV-2 has successfully infected a human cell, it multiplies rapidly.
We are now experiencing a novel pandemic. In general, people are actively working in unprecedented ways to stop SARS-CoV-2 from spreading. One way to bring our own lives into context and insight is to look at the historical record. Unfortunately, the cessation of influenza in the summer of 1919 does not indicate that COVID-19 would eradicate in 2020.
The technical aspects of the pandemic are daunting obstacles. They are playing out in a global economy that has come to a halt, with growing demands to reopen societies and create a technologically advanced and integrated society – all problems that our forefathers did not have to consider a century ago.
This article co-writes by Jessica Pickett, Ph.D., a principal consultant with Tomorrow Global, LLC.