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When a Pandemic Becomes Endemic

Bunnany Pekar, PhD, ARNP, CRNA

On January 30th, 2020, the World Health Organization (WHO) declared the outbreak of SARS-COV2, the virus that cause Covid-19 (CV-19) a public health emergency of concern.  The current global pandemic started on March 11th, 2020. Since then many lives have been lost. Millions of people still suffer from morbidities associated with recovery from CV-19.  As of January 31st, 2022, there are approximately 370 million cases and over 5.6 million people succumbed to the disease.  Over 74 million people have been affected by C19 in the United States. More than 900,000 people have died. This article will clarify when a pandemic becomes endemic, and how we will get there.

Different Variants

Since discovery, the SARS-COV2 virus have mutated to many different variants.  The WHO classified these variants into variants of interest (VOI) and variants of concerns (VOC).  The VOIs were Lambda and Mu.  The VOCs were Alpha, Beta, Gamma, Delta, and the latest one, Omicron. 

The Omicron variant has been the WHO’s VOC since November 26th , 2021 when it was regarded to be highly contagious. This is either due to immune evasion, increased transmissibility, or both.  A Newsweek article published on January 18th, 2022, predicts that approximately 1.5 million people may be affected by the Omicron surge. Even worse, around 190,000 may be hospitalized.  This data accounts for a time span between mid-December to mid-March 2022 – when Omicron infections are expected to subside.  The same model predicts that deaths from this variant surge could be between 58,000 to 300,000.  As of the January 28th, 2022, the US is about 130,000 deaths from the 1 million mark since the start of this pandemic.

However, even with the current rise in rates of infections and hospitalizations, there has been a lot of discussion about when this pandemic may be over. This indicates that the disease shifts from pandemicity to endemicity.  

The thought of this horrible disease becoming “just a part of everyday life” seems frightening given the amount of emotional, psychological, and financial impact it has inflicted on society thus far.  However, the world had experienced many pandemics and some of those viruses continue to exist as part of everyday life.  Before we discuss how a pandemic becomes an endemic, here is a review some epidemiological terms.

When a Pandemic becomes endemic
(Source:  Image by Wellcome Trust, retrieved from When will COVID-19 go from pandemic to endemic? | World Economic Forum (weforum.org)

Disease States

Epidemic

An epidemic refers to a disease with rates that are clearly above the expected occurrence in a community or region. Examples can include smoking and vaping.

Pandemic

However, when a disease grows to an exponential rate, it becomes a pandemic.  Pandemicity refers to when there is a sudden increase in cases cross many countries or worldwide. 

There have been many pandemics that have occurred in history. However, no illness in recent history has affected the entire world like CV-19.  Here are a few notable pandemics:

Swine Flu

In 2009, the H1N1 virus, also known as the “swine flu” took the lives of approximately 12,000 Americans.  The virus is still present during flu season.  SARS, caused by the virus, SARS-COV1, is the first pandemic of the 21st century (2003).  A type of type of coronavirus, it had spread over four continents.  There have been no new cases of SARS-COV1 since mid-2003. However, it is still an infectious agent with the potential to have a devastating effect on population health. SARS affected approximately 8000 people, with about 700 deaths. Only 8 cases were confirmed in the United States.

H3N2

In 1968, the H3N2 influenza A killed nearly 1million people word wide including 100,000 Americans.  This was an influenza virus with two genes from avian flu strains. The virus continues to mutate and circulate during flu seasons.  The H2N2 (1957) referred to as the “Asian flu” killed approximately 116,000 people in the US. In total it killed approximately 1.1 people worldwide.  The deadliest pandemic was the H1N1 influenza pandemic of 1918. Roughly one third of the world’s population contracted the virus. This killed 50 million people globally. Another 675,000 in the United States alone. 

Endemic State

When the rate of infection declines, the virus – and its associated disease – will be considered endemic. This means that it is always present in a particular region. In some economically underdeveloped countries where water treatment facilities are insufficient, cholera is endemic. In rural parts of Spain, recurring tick-borne fevers are endemic.  Malaria is still an endemic disease in 21 countries.  

Endemic diseases can still have an occasional unexpected spike in the number of cases of the same illness in an area. There may also be an appearance of a new disease in a geographical area.  This is the definition of an outbreak.  The terms outbreak and epidemic are interchangeable. However, an epidemic is more widespread.  The Ebola virus that spread within three African countries from 2014-2016 is considered an epidemic disease.  An outbreak, however, does not have to be a particular disease; it could be a behavior leading to poor health such as vaping-related lung injuries.  Diseases that have an irregular outbreak pattern are considered to be sporadic.  

The Omicron variant outbreak is considered to be highly transmissible but less virulent. Scientists have projected that the disease may shift from pandemicity to endemicity.  

This shift does not mean that the virus disappeared; instead, it occurs when enough people gain immune protection either from vaccination or natural infection such that there is less transmission and less CV-19 related hospitalizations and death, even when the virus continues to circulate.  

Virus Spread

Viruses spread when there are enough susceptible hosts and enough contact to sustain the spread. To continue with replication and inoculation, a percentage of the population must be capable of getting a disease, also known as threshold proportion. However, if the proportion of the population that is immune to the disease is greater than this threshold, the spread of the disease will decline, known as the herd immunity threshold.  This threshold varies from disease to disease, meaning that the more contagious a disease is, the greater the proportion of the population that needs to be immune to the disease to stop its spread. Disease such as measle is highly contagious and requires approximately 94% of the population to be immune in order to interrupt the chain of transmission.  

With the Delta variant, the basic reproduction number, Ro (R naught), is around 5.08, much higher that the initial SARS-COV2 from China, where the Ro is approximately 2.79.  Omicron is highly transmissible with an Ro approximately 7.0 or greater, meaning that one case of Omicron infection can result in more than seven other infections.  As a matter of perspective, the Ro for seasonal flue is around 1.2, while measles has an Ro of 12-18!

Predictability

As you can see, it is hard to predict when this transition to endemicity will occur.  Many factors must be considered. These include:

  • strength and duration of immunity
  • social behaviors
  • transmissibility of the virus
  • emergence of new variants of the virus

Shortly after the emergence of the Omicron variant (BA1), scientists have discovered a new variant of this variant.  This new Omicron variant, BA2, also known as the “stealth variant,” has already spread across Europe and has been detected in the US.  This concerning as it could be more transmissible than the original Omicron, leading to more CV-19 cases, further burdening an already taxed healthcare system. 

Despite that, the CV-19 pandemic becoming an endemic disease is a real possibility.  However, there are many unknowns.  While there have been more vaccinations with the surge of the Omicron variant as well as those contracting CV-19, it is unclear how long immunity to the disease will last.

The Future?

The preferred intervention for combating most infectious disease is through vaccination because of the potential risk of morbidity or mortality of exposing an individual to the real disease.  For the SARS-COV2, vaccination during the epidemic/pandemic phase is essential to relieve the burden of disease particularly those that are vulnerable.  Vaccination has both direct and indirect effects:  reduction in susceptibility, infectiousness, and pathology of the vaccinated person and reduction in the probability that an unvaccinated person will become infected over time.  Unfortunately, immunity either through infection or vaccination is not life-long due to the nature of the coronavirus, unlike the measles, chicken pox, or rubella viruses, where infection or vaccination induces life-long immunity. It is likely that COVID-19 will remain in an endemic disease state long-term.  

Human coronaviruses (hCOVs) are single-stranded RNA viruses. Most replicate in the upper and lower respiratory tract.  The age of onset of infection is around 4 years old, where most children have turned seropositive for IgG antibodies to hCOVs.  The infection appears like the common cold and elicits both antibody and T cell immunity peaking after this illness.  Studies suggest that individuals are refractory to re-infection shortly after an infection. However, protection starts to weaken over time. This explains why people get infected more than once.  It appears that the hCOVs and SARS-COV2 have similar immune features.

As we transition from pandemic to endemic state, we may need continued vaccinations and public health practices such as social distancing, masking indoors, and hand hygiene to keep the virus at bay.  

References:

Antia, R., & Halloran, M. E. (2021). Transition to endemicity: Understanding COVID-19. Immunity, 54(10), 2172–2176. https://doi.org/10.1016/j.immuni.2021.09.019

Epidemic, Endemic, Pandemic: What are the Differences? (2021).  Retrieved from Epidemic, Endemic, Pandemic: What are the Differences? | Columbia Public Health

Enhancing readiness for Omicron.  The Word Health Organization.  Retrieved from 2021-12-23-global-technical-brief-and-priority-action-on-omicron.pdf (who.int)

Frequently asked questions:  SARS.  Retrieved from SARS | Frequently Asked Questions | CDC

Heard immunity and Covid-19:  what you need to know.  The Mayo Clinic.  Retrieved from Herd immunity and COVID-19 (coronavirus): What you need to know – Mayo Clinic

Omicron is the dominant variant for two reasons.  Boarman, A.  (2021).  Retrieved from Omicron is the Dominant COVID Variant for Two Reasons | Vitals (sutterhealth.org)

Pfizer executives say Covid could become endemic by 2024.  (2021).  Retrieved from Pfizer executives say Covid could become endemic by 2024 (cnbc.com)

The omicron variant has its own ‘stealth’ variant. Here’s what to know.  Scribner, H. (2022).  Retrieved from Omicron stealth variant: What is it? What is the mysterious BA.2? – Deseret News

Tracking the SARS-COV2 variants.  The World Health Organization.  Retrieved from Tracking SARS-CoV-2 variants (who.int)

U.S. Could See Over 1 Million More Hospitalizations Before Omicron Subsides.  Newsweek.  Retrieved from U.S. Could See Over 1 Million More Hospitalizations Before Omicron Subsides (newsweek.com)

What to know about BA.2, the newest Covid omicron variant.  NBC News.  Retrieved from What to know about BA.2, the newest Covid omicron variant (nbcnews.com)

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