Updated: December 31, 2021 9:09:55 am
We do not have a precise timeline, but past pandemics offer tantalising clues. The widespread use of dashboards has created the perception that the pandemic will end when all dashboard indicators hit zero (infections, cases and fatalities). However, history suggests this is the most unlikely outcome.
In the past 130 years, respiratory pandemics have been followed by annual seasonal waves fuelled by viral endemicity, which generally lasts until the next pandemic. What goes down, comes back up. The term “waves” to refer to patterns of disease spread during an outbreak was first used during the Russian Flu pandemic of 1889. It lasted three years, had multiple phases of spikes and valleys with the second phase being the most severe.
The Spanish Flu of 1918-20 had three distinct peaks. It began as a small wave in March 1918, which subsided during the summer. Following the initial peak in cases, a larger peak occurred in the fall of 1918. A third peak occurred during the winter and spring of 1919. This wave subsided in the summer of 1919, signalling the end of the pandemic. It is estimated that over 500 million people were infected and about 100 million died. Although the pandemic subsided, the viruses didn’t go away; a descendant of the Spanish Flu virus, the contemporary H1N1, is circulating even today.
Multiple factors contribute individually or in combination to determine disease patterns. Some diseases are seasonal, and the waves follow seasonal patterns. Human behaviour and interactions also affect viral spread. In India, during the monsoons, due to crowding, a spike in vector-borne diseases (dengue and malaria) is common. Lifestyle choices, too, play a role — school closings during summer and winter have been linked to reduced social contacts, and thereby reduction in influenza cases.
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A third factor that influences disease spread is the level of immunity in the community (herd immunity). As more individuals gain immunity either through infection and/or by vaccination, they indirectly offer protection to those who are not infected. The disease spread slows and eventually halts as the virus is unable to find new hosts to infect.
The epidemiology of previous deadly coronaviruses (SARS-CoV-1, and MERS-CoV) differs significantly from that of SARS-CoV-2; hence these pathogens are not helpful models for predicting the future of the Covid-19 pandemic. Influenza pandemics are our best comparative models. At least eight influenza pandemics have occurred since the early 1700s. Seven of them had an initial peak that faded away without any major intervention over the course of a few months. Following that, around six months after the initial spike, each of those seven pandemics experienced a second significant peak. After the initial wave of infections, certain pandemics displayed recurrent smaller waves of cases over the next two years. The 1968 pandemic was the only one to follow a more classic influenza-like seasonal pattern, with a late fall/winter spike followed by a second surge the following winter. The second year saw an increase in pandemic-related mortality in various locations, notably in Europe.
The course of these pandemics was not significantly influenced by vaccination. There were no flu vaccinations available in 1918. When the H2N2 epidemic swept the globe in 1957, flu vaccination was primarily used by the military. The United States developed approximately 22 million doses of vaccine during the 1968 pandemic of H3N2, but by the time it was available, the worst of the epidemic had gone. Yet, vaccines play a crucial role in controlling mortality and the burden on healthcare.
A combination of herd immunity and the virus mutating to become less infectious and severe led to the eventual end of past pandemics. Usually, viruses don’t just go away. Following three pandemics since 1900, the influenza A strain mutated to become increasingly human-adapted and eventually displaced the dominant seasonally circulating influenza virus. Viruses descended from the 1918 virus have caused almost all instances of influenza A since, as well as all subsequent flu pandemics. Seasonal flu continues to result in the deaths of 6,50,000 people each year.
SARS-CoV-2 will most likely become endemic and continue to circulate in the human population synchronising to a seasonal pattern with less severity over time, as other less pathogenic coronaviruses, such as the OC43, 229E, NL63 and HKU1 and past pandemic influenza viruses have done.
One of the probable scenarios is that through vaccination or natural immunity the adult population will develop immunity and only contract mild illness. SARS-CoV-2 will then primarily affect young children, who will probably only have mild illnesses. The duration of immunity against SARS-CoV-2, both natural and through vaccination, is still uncertain. After six to eight months, individuals who have had Covid-19, show declining levels of neutralising antibodies. However, their bodies also produce memory B cells, which may develop antibodies in the event of a reinfection, and T cells, which can kill virus-infected cells.
SARS-CoV-2’s destiny will also be determined by whether or not it spreads to wild animals. Several illnesses that have been controlled continue to exist because animal reservoirs allow infections to spread back into humans. Yellow fever, Ebola, and the chikungunya virus are examples of these diseases. Many animals, including cats, rabbits and hamsters, are susceptible to SARS-CoV-2. Mink are particularly susceptible to Covid-19, and outbreaks have occurred on mink farms in Denmark and the Netherlands.
Omicron has rapidly become the prevalent variety in many countries. In their modelling study, the US-based Covid-19 Scenario Modelling Hub predicts a significant wave of Covid-19 cases that, by the first week of January 2022, will surpass those seen nationwide during the height of the Delta wave. While it is evident that the infection wave will be considerable, it is less clear what Omicron’s effect will be in terms of hospitalisations and fatalities, as much is unknown about the severity of primary, secondary, and breakthrough infections with Omicron compared to Delta and preceding variants. Regardless of where Omicron’s relative severity falls, the sheer number of cases expected implies that even a relatively mild Omicron variation has the potential to significantly stress, if not overwhelm, already overburdened healthcare systems. We are yet to see if Omicron is a step towards endemicity or one away from it.
The hope for targeting SARS-CoV-2 eradication has passed. Transitioning to a post-pandemic environment is not likely to be one with “zero-Covid”. The challenge then is to define the Covid-19 level that is acceptable for countries in a world that is fundamentally interconnected.
The epidemiological characteristics of a pandemic’s end are not universally defined. Prior respiratory pandemics illustrate that ends are usually ambiguous, and that pandemic closure is better viewed as the return of social life rather than the attainment of specific epidemiological goals. Pandemics end not when disease transmission ceases, but rather when the disease ceases to be newsworthy in the eyes of the public and in the judgement of media and political leaders who mould our attention and opinions. The end of a pandemic is more of a concern of lived experience than of biology, making it a social rather than a medical event.
The writer is director, Medical Research and Hon. Neuropsychiatrist, Jaslok Hospital & Research Centre, Mumbai. He has co-authored The Coronavirus: What You Need To Know About The Global Pandemic
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