We are still in the midst of the worst pandemic in a century that began relatively quietly in late 2019 with the early rumblings of pneumonia cases of unknown etiology in China. A pandemic, caused by the coronavirus SARS-CoV-2 (a novel virus isolated in January 2020, but novel no longer), that has led to more than 500,000,000 cases and over 6,000,000 deaths worldwide, likely an underestimate. At home in Mississippi, almost 800,000 cases and 12,000 plus deaths have been reported, again, without a doubt an undercount. The World Health Organization (WHO) first declared the initial outbreak of SARS-CoV-2, and the illness that it causes, Coronavirus Disease 2019 (or COVID-19), a Public Health Emergency of International Concern on January 30, 2020, followed by a pandemic declaration on March 11, 2020.1 Coincidentally, March 11, 2020 is also the day the Mississippi State Department of Health (MSDH) reported the first case of COVID-19 in a Mississippi resident. Since that time the pandemic has affected every aspect of our lives, certainly the lives of public health and medical professionals.
We have all experienced the ups and downs of the pandemic acutely, from our early successful efforts to “flatten the curve”, the first big wave fueled by returning college students and Fourth of July get togethers (among other activities) in the summer of 2020, the emergence of a seemingly never ending Greek alphabet of variants and sub-variants, the severity of illness from the Delta variant leading to thousands of deaths including pregnant women and children, the enormity of cases and pressure on hospital capacity from the Omicron variant surge, and now to the relative low number of cases, hospitalizations and deaths we are currently experiencing.
Does the current low incidence we have been experiencing signal COVID-19 is becoming endemic, and we are returning to normalcy? Dr. Anthony Fauci has publicly indicated that we certainly seem to be out of the pandemic phase in the US; we are not seeing the overwhelming number of cases, hospitalizations and deaths that have been the hallmark of COVID-19.2 While I tend to agree that we are indeed not seeing the broad transmission and the impact we have seen to date in the US, and in Mississippi for now, with the widespread transmission that persists throughout the world, we do remain in a pandemic. And there is an expectation that there will be additional periods of increased transmission as we move through the next months, though perhaps to a lesser extent and with fewer severe outcomes.
Endemic versus Pandemic
The term pandemic may not have one agreed upon definition, especially considering the many debates in the media (and social media), and among scientists and health agencies, about what constitutes a pandemic. The demarcation for when an infectious disease that has led to a pandemic is considered no longer pandemic and becomes endemic may also not be exactly clear-we may just know it when we see it.
However, let’s look at the differences between a pandemic and the endemic occurrence of diseases.
The amount of a disease that is usually present in a community is referred to as the baseline or endemic level of the disease.3 While the baseline level of the disease may in fact be higher than we would like, the baseline or endemic level is what is observed. For example, in Mississippi many STIs are endemic but certainly occur at higher incidence than desired.
Epidemic refers to increases above the expected baseline. These may occur as somewhat predictable seasonal increases, such as West Nile Virus or influenza. But an upsurge may also be termed an outbreak, such as when cases are grouped in a geographic area or a defined timeframe, even when there is not an expected baseline occurrence.
A pandemic is an epidemic with widespread transmission of a disease leading to a substantial number of cases in all parts of the world or multiple continents simultaneously.4 But there are other components other than broad geographic distribution that are associated with a pandemic.4 Typically, pandemics are caused by a new emerging infectious agent, usually a novel virus. But a novel virus alone does not always lead to a pandemic. A key component in the process is contagiousness, primarily through rapid sustained human to human transmission (or other mechanisms such as waterborne or through animal vectors). SARS-CoV-1 and Middle East Respiratory Syndrome (MERS), both emerging coronaviruses, together caused sporadic cases but did not lead to a pandemic because they did not develop (or have not yet developed) the critical characteristic of sustained transmission. Also, because an emerging virus is new there is minimal to no population immunity, further facilitating widespread propagation. What is not necessarily implied by the term pandemic is severity of disease which can occur with some variability.
Transition from Pandemic to Endemic
While none of us have a crystal ball to predict when the move from pandemic to endemic will occur or what exactly that will look like, there are a few aspects that are likely. Predicting the future is a dicey proposition at best and will make many of my public health mentors cringe, but most of the assumptions outlined are within reason.
Disease Incidence: So, what might the occurrence and impact of COVID-19 look like in the coming months as we potentially shift to endemic levels? With a transition to endemic COVID-19, we should expect cases to continue to occur year-round, punctuated by periods of epidemic increases over the baseline. These epidemic increases may fall into some level of seasonality. There certainly is a precedent for this with many respiratory viruses, including influenza and other human coronaviruses. In fact, we may already be seeing a somewhat seasonal pattern with COVID-19, with surges in the summer and winter months since July 2020. Seasonal variations are common for other infectious diseases as well (summertime increases with bacterial enteric diseases are typical, for example), though we don’t fully understand all the mechanisms that lead to these variations. Ultimately, we may also see variation in the seasons between the northern and southern hemispheres, again much like we do with other respiratory viruses.
But SARS-CoV-2 virus has continued to provide surprises as the pandemic has evolved, and predictions of future incidence and seasonality can be challenging due to the emergence of variant and sub-variant strains that lead to changes in both virulence and infectiousness. We should expect to see variants develop with regularity, and this will impact the likelihood of epidemic increases, regardless of the time of year.
In a recent study published by the Centers for Disease Control and Prevention (CDC), the overall seroprevalence (proportion of the population with SARS-CoV-2 antibodies) in the US during the Omicron surge was estimated at 57.7%.5 It was even higher for children aged 0-17 years, with an estimated seroprevalence of approximately 75%. Additionally, 66% of the US population and 52% of the Mississippi population have been fully vaccinated. Therefore, is likely that a substantial proportion of the population has developed either natural or vaccine induced immunity, or both. While this immunity may wane over time limiting the effectiveness at preventing infection, we can anticipate some attenuation of the severity of disease, even with breakthrough infections. This is a big step towards limiting the impact of future surges and brings us closer to endemicity.
However, even if baseline incidence and seasonality prove predictable, which allows for coordinated planning, response and resource allocation, seasonal variations may lead to pressure on healthcare capacity and ultimately to deaths. Even influenza, which the public often dismisses as a non-serious infection, leads to anywhere from 12,000 – 52,000 deaths annually according to the CDC.6
Vaccination: The rapidity of development and Emergency Use Authorization by the FDA of COVID-19 vaccines has been one of the big success stories in our efforts to control the spread and impact of infection. The vaccines based on mRNA technology (Pfizer and Moderna) have proved to be the most effective. However, the complicated schedule of additional doses for immunocompromised individuals, and eligibility for first and second booster doses based on prior vaccine type and with varying timeframes, have proved confusing to both the public and healthcare providers. Moving forward, simplification of the vaccine schedule will be important. We may see recommendations ultimately shift to a more standard schedule, such as a dose every several months based on waning immunity or seasonality. As variant strains develop, variability in vaccine effectiveness may be apparent, but even when the vaccine is not a close match to the circulating variants, we hopefully will see vaccines continue to provide protection from the development of severe disease.
Surveillance and Public Health Response: Surveillance, much of it based on individual case reporting and case investigation, and the daily public data reporting for almost every aspect of the pandemic, have been key components of the public health response. Management and distribution and administration of vaccines and tests/testing supplies, PPE, and therapeutics through the Mississippi State Department of Health (MSDH) and other state agency partners have been a necessity to oversee limited resources and rapid dissemination.
Moving forward, a more sustainable model for both will be a need. Some important steps for surveillance sustainability have already been taken, such as a less frequent public reporting cycle and a priority for disease outbreak investigation rather than on individual cases, especially in highly vulnerable congregate settings. Influenza again serves as a model, and while positive test reports through electronic means will remain important, syndromic surveillance to monitor trends in individuals seeking care for COVID-like Illness (CLI) and robust virologic surveillance to identify emerging variant strains will be cornerstones as we shift to the endemic phase. Hospital based and death surveillance as indications of severity of disease will remain vital to direct public health actions. Some intriguing additional surveillance methods we have already begun to explore are wastewater surveillance and self-reporting of positive tests. All of these are tools to help inform incidence, impact and predicting the next wave(s).
With the change in public health responses, the model for distribution and administration of COVID-19 vaccine, availability and location of testing and evaluation, and distribution and prescribing of medical therapeutics should mirror a more traditional healthcare model. Vaccine, testing supplies and therapeutics are no longer in short supply. Using all our available tools without complex distribution systems will be a necessity in the endemic phase.
Are we done?
In looking at the definition of pandemic above, we can anticipate the pandemic will end when the virus is no longer prevalent throughout the world or in multiple countries and regions. A medical end of the pandemic is considered when a vaccine or effective treatment is developed (we have both) or when infection and death rates significantly drop (in the US and MS for the moment). Another end to the pandemic may be a social end, when people tire of the pandemic and the public health recommendations to prevent infection – in this case not an actual end of the disease and may delay a medical end.
In many instances, folks have been going about their business without following masking recommendations and avoiding large gatherings for quite some time. They haven’t been getting their children or themselves vaccinated or boosted. Immunity is high from the Omicron wave now, but we do know immunity can wane.
We may be tired and done with the COVID-19 pandemic, but COVID-19 is not done with us quite yet. We are, however, closer to the end of the pandemic than we have been at any time in the last almost 2 ½ years, and the lower number of cases, hospitalizations and deaths have been welcome. I remember the early days of response to the spread of COVID-19 vividly (some of you may recall 2019-nCoV—the very early moniker), with the understanding at that time that what we were about to experience would be a defining moment in our lives. Certainly, it has been in mine, both personally and as a public health professional. None of us could have predicted the storm of events or the length of the pandemic. The tragedy of COVID-19 associated deaths, especially in a child, is not an experience I will soon forget.
As I write this, MSDH surveillance is indicating increases in cases and outbreaks due to COVID-19. Let us stay the course and remain vigilant.