For public health officials, the constantly mutating COVID-19 disease is a fast-moving target. As new variants emerge and spread rapidly, epidemiologists must scramble to catch up. Some variants, such as Delta, cause a deadlier form of the disease. Others, such as Omicron, are highly contagious but cause milder illness. Either way, epidemiologists must work fast to stay ahead of each new variant.
COVID-19, influenza, and AIDS are just a few modern examples of viruses that have plagued humanity throughout history. Epidemiologists are at the forefront of the battle against these diseases, using both time-tested (quarantine) and innovative (genetic epidemiology) tactics. Understanding what viruses are and how viruses mutate is part of an epidemiologist’s important role.
What Are Viruses?
Viruses are microorganisms of genetic material that are dependent on living organisms as hosts. Once they infect, or take over, a host, they hijack the host cell to reproduce. Viruses have some of the features of a living entity in that they can reproduce and mutate. However, they can’t move on their own or survive outside of a host body.
Types of Viruses
Viruses consist of genetic material surrounded by a protein coat called a capsid and a mix of lipids and proteins that surrounds the capsid called the “envelope.”
Two main classes of viruses exist: RNA viruses and DNA viruses. RNA viruses consist of a single or double strand of RNA. They use this genetic code to reproduce and mutate far more rapidly than their counterpart DNA viruses. The rapid mutations of RNA viruses make it difficult to develop vaccines against them.
DNA viruses contain a double strand of DNA. They are more stable than RNA viruses, more likely to self-correct, and don’t mutate as often, making it easier to develop vaccines against them.
Viruses are also categorized by shape.
- Helical. An example of a helical (spiral shaped) virus is a plant virus, such as the tobacco mosaic virus (TMV).
- Polyhedral. A respiratory virus, such as an adenovirus, is an example of a polyhedral (many sided) virus.
- Spherical. Coronaviruses, such as those that cause COVID-19 or severe acute respiratory syndrome (SARS), and the common cold, are spherical viruses.
- Complex. Bacteriophages are complex viruses. They consist of many different shapes and structures.
How Do Viruses Work?
Viruses use an organism’s cells to survive and reproduce. They travel from one organism to another. Viruses can make themselves into a particle called a virion. This allows the virus to survive temporarily outside of a host organism. When it enters the host, it attaches to a cell. A virus then takes over the cell’s reproductive mechanisms for its own use and creates more virions. The virions destroy the cell as they burst out of it to infect more cells.
Viral shedding is when an infected person releases the virus into the environment by coughing, speaking, touching a surface, or shedding skin. Viruses also can be shed through blood, feces, or bodily fluids.
Viruses and the Human Body
Viruses are often confused with bacteria, but they’re very different entities. Bacteria are living organisms that can move; reproduce; generate their own energy; and live in the air, water, and soil, as well as inside another living organism. In fact, because bacteria are living organisms, viruses can infect them. Such viruses are called phages, or bacteriophages (Greek for “bacteria eater”).
The human body hosts about 380 trillion viruses, or virome. Scientists have connected some of these viruses to a number of different illnesses, such as periodontal disease and cervical cancer (human papillomavirus, or HPV). Others, however, simply coexist as part of the human biome, which consists of bacteria, yeast, and other organisms. Still, others may have a beneficial effect against disease.
Viruses and the Immune Response
When a virus infects a person, the immune system mounts a response. This defense reaction generally begins with white blood cells, which are called phagocytes and leukocytes. These cells attack viruses and bacteria and devour them. If the immune system recognizes a virus from a previous infection or a vaccine, the body triggers immune responses specific to the strain of the virus. The response helps to produce antibodies, which can protect the body from further infection.
However, it’s essential to note that some viruses are more adaptive than others, which is why certain vaccines are recommended at closer intervals. For example, an RNA virus such as Influenza can quickly mutate away from its original strain, requiring a new flu vaccine to be created every season.
All About Viruses: A Resource Guide
Learn more about viruses and the diseases they cause and the public health response through the following resources:
Idaho Public Television, Viruses: Facts: Learn about the basics of viruses, including how they travel between hosts and how the human immune system fights them.
Centers for Disease Control and Prevention, Key Facts About Seasonal Flu Vaccine: Flu is an RNA virus. It mutates fast. Scientists have to develop a new flu vaccine yearly to work against each season’s strain.
Johns Hopkins Medicine, “COVID-19 Vaccines: Myth Versus Fact”: How does the COVID vaccine work? What are the common misconceptions? Experts at Johns Hopkins Medicine discuss the facts and myths around the COVID shot.
SciShow, “Are Viruses Alive?”: Viruses reproduce and evolve — so, are they alive or aren’t they?
Why Do Viruses Mutate?
Mutations can be a death sentence for a virus, causing it to evolve itself out of existence. So, why do viruses mutate considering that the process leads to a dead end?
Mutation is a natural process for survival. By becoming more effective in moving from host to host and reproducing faster, a virus can extend its life. Other natural reasons for mutation include becoming more effective in adhering to host surfaces, such as the spike protein of COVID-19. Mutation also helps viruses to evade immune responses and vaccines.
Sometimes viruses mutate as they make copies of themselves. Sometimes reproduction causes errors, and a gene reproduces incorrectly.
Sometimes these errors have no impact at all. Often, however, these errors, or mutations, can be beneficial. If the changes are heritable, they’re passed down to future generations.
All organisms mutate. Mutations are the basis for evolution and natural selection. Virus mutation is one of the most compelling arguments for viruses to be classed as living organisms. As with any mutation, some changes will provide a benefit that leads to more efficient reproduction. Others may be dead ends or even harmful, limiting an organism’s ability to thrive.
What Causes Viruses to Mutate?
As with organisms, every time viruses replicate, they introduce more room for error. Time increases the chance of a replication error. Other causes of mutation include the host environment. A person with a weakened immune system who’s infected with a virus may not be able to fend it off as easily, allowing the remaining virus to adapt to its new environment. When it escapes, it can more easily infect another person.
Recombination, in which two variants infect the same cell, can also cause advantageous mutations, potentially making a virus stronger and more difficult to fight.
Benefits of Mutation
Several benefits can explain why viruses mutate. Although many mutations are dead ends, viruses that successfully adapt to their host environment are better equipped to survive and spread. Each successful mutation creates a variant that’s more difficult to counter.
- Faster spread. Some mutations allow viruses to move faster between hosts.
- Less vulnerable. Coronavirus variants can partially escape the immune response, making vaccines and the body’s own defenses less effective.
- More robust. Viruses have to be able to survive outside of a host long enough until their next ride comes along. Cold viruses, for example, can live for several hours on a surface.
- Vaccine resistance. Different strains can prove to be more resistant to existing vaccines, as the yearly flu strain illustrates. However, vaccine research has begun to make inroads into this challenge with mRNA technology, which can speed up the process of developing vaccines that are effective against different virus strains.
Mutations, Variants, and Strains
Not all mutations cause variants and strains. Below are definitions that explain how mutations, variants, and strains differ.
- Mutation. Mutations are errors in the replication of the virus’s genetic code. Mutations can be beneficial to the virus, deleterious to the virus, or neutral.
- Variants. Viruses with these mutations are called variants. The Delta and Omicron variants are examples of coronavirus mutations that cause different symptoms from the original infection.
- Strains. Variants that have different physical properties are called strains. These strains may have different behaviors or mechanisms for infection or reproduction.
How Quickly Do Viruses Mutate?
How viruses mutate is entirely random. However, mutation speed can depend on many factors:
- Type of virus. Whether the virus is an RNA or DNA virus will impact how fast and how often it mutates.
- The environment. Viruses react to pressures from the host and may mutate according to their external surroundings.
- Antigenic drift. With flu viruses, mutations add up over time, changing the viruses’ surface proteins, making them unrecognizable to the immune system.
- Antigenic shift. An antigenic shift is a sudden change in the flu virus, in which the segments from two different viruses combine into a novel strain. An example is the swine flu virus (H1N1), which caused a pandemic in 2009.
What Are RNA Viruses?
The three most well-known RNA viruses are coronaviruses, the virus that causes AIDS, and the flu virus. Because of how quickly the viruses mutate, scientific research into vaccines and antiviral drugs has had to delve deep into the genetic code and the mechanisms for how RNA viruses reproduce and spread.
SARS-CoV-2
SARS-CoV-2 is an RNA coronavirus. It’s in the same class of viruses as SARS-CoV, which caused SARS in 2002 and Middle East respiratory syndrome (MERS) in 2012. These viruses jumped from animals to humans.
With SARS-CoV and SARS-CoV-2, the most likely original hosts were bats. Camels are believed to be the original or intermediary hosts for MERS. Scientists developed COVID vaccines on an mRNA mechanism, which uses a piece of genetic material to prompt the immune system to recognize the virus and attack it. This technology is being tested in vaccines to treat AIDS.
Flu Viruses
A case of the flu is potentially lethal. An infamous worldwide influenza pandemic in 1918 caused a death toll of an estimated 500 million people. The flu virus has two main types: influenza A and influenza B. Each year, different strains arise. Scientists develop vaccines based on the expected dominant strain that’s circulating according to epidemiology data from public health organizations. Globally, the flu causes 290,000 to 650,000 deaths each year.
HIV-1
The human immunodeficiency virus (HIV) causes AIDS. As with SARS-CoV-2, it crossed over to humans from animals, likely from chimpanzees. HIV is a retrovirus, because of the way it hijacks cells and replicates inside of them. AIDS drugs inhibit this process. However, HIV mutates rapidly, causing drug-resistant strains that can spread to others.
HTLV-1
Since viruses take over the reproductive engine of cells, they’ve been connected to certain cancers. That is the case for human T-cell lymphotropic virus 1 (HTLV-1), which is also a retrovirus. HTLV-1 has been linked to leukemia and lymphoma.
Advantages and Disadvantages of RNA Viruses
RNA viruses are difficult to understand, in part because of their constant mutation. How these viruses mutate protects them from the immune system attacks of their hosts. These rapid mutations (up to a million times faster than their hosts) pose challenges to developing vaccines and antiviral treatments. For example, drug resistance in HIV patients means that over time, these drugs are no longer as effective in treating AIDS.
On the other hand, RNA viruses are prone to mutagenesis, or mutations that can be disastrous for the viruses. Faster mutations mean the chance for more errors, causing viruses to mutate themselves out of existence. Scientists developing antivirals are investigating a strategy called lethal mutagenesis to drive RNA viruses to mutate themselves into extinction.
What Are DNA Viruses?
DNA viruses replicate by entering the nucleus of the cell, and then copying their DNA code over the cell’s DNA. Types of DNA viruses include the viruses that cause herpes, Epstein-Barr, and smallpox. Below are examples.
Herpesvirus (HSV and HHV)
The herpesvirus family includes several viruses that cause a wide variety of diseases. These include genital and oral herpes, eczema, and shingles and chickenpox (not related to smallpox). Herpesviruses are characterized in part by their ability to create latent infections that are triggered under certain conditions.
Orthopoxvirus
Orthopoxviruses cause smallpox, monkeypox, and cowpox. These diseases are characterized by infectious skin lesions. Smallpox is severe and often deadly. However, it’s been eradicated due to a worldwide vaccination program. In rare cases, cowpox can cause an orthopoxvirus infection via inoculation.
Adenovirus
Adenoviruses cause coughs, colds, diarrhea, and pink eye. Children are the most prone to get and spread these infections, especially in schools and camps. Adenoviruses are highly contagious but usually cause only mild symptoms. However, they may be more serious in individuals who are immunocompromised.
Advantages and Disadvantages of DNA Viruses
One difference between DNA and RNA viruses is how quickly they mutate. DNA viruses are much more stable and mutate at a much slower rate than RNA viruses.
Because DNA viruses replicate inside the DNA of the host cell, scientists have been developing applications to use this power for good. They’ve been working to modify the genetic material of DNA viruses and insert them into cancer cells; this could trigger the immune system to mount an attack.
SARS-CoV-2 and Mutations
While it may seem as if the number of COVID-19 variants seems to be increasing faster than the world can fight them, the SARS-CoV-2 virus mutates four times slower than seasonal flu. The reason is that even though SARS-CoV-2 is an RNA virus, it has what scientists call a proofreading mechanism. This chemical process helps prevent errors in replication, similar to the way a proofreader prevents errors from appearing in the text. This extra step slows mutations and prevents mutagenesis. On the plus side, it gives scientists an edge when developing vaccines.
COVID-19 vaccines are more likely to be somewhat effective against later strains and variants, even if they don’t provide full immunity. Further, immunity from previous infections can be more effective against later infections.
The Science of Virus Mutations: A Resource Guide
All viruses mutate. That’s what makes them deadly, beneficial, and fascinating. Below are helpful resources for learning more about how viruses mutate and the science behind mutation.
Abbott, Virus Hunters Map How COVID-19 Variants Travel: This resource shows how scientists trace the lineage of virus variants to learn how they spread and understand what makes them evolve.
American Council on Science and Health, “The Idiot’s Guide to Viral Mutation”: This laypersons guide provides an overview of the science behind viruses and mutations.
Centers for Disease Control and Prevention, Understanding Variants: This article provides an overview of what virus variants are, how they may become resistant to vaccines or treatments, and how people can protect themselves from getting diseases.
U.S. Food and Drug Administration, SARS-CoV-2 Viral Mutations: Impact on COVID-19 Tests: This article explains how mutations of the SARS-CoV-2 virus affect the accuracy of COVID tests.
What Is the Epidemiology of COVID-19?
Ever since the World Health Organization (WHO) alerted the world to the appearance of a SARS-like illness in Wuhan, China, epidemiologists have been on the case. Epidemiology is the study of health and disease in specific populations. Epidemiologists are scientists who look at the spread of disease, its causes, and risk factors. They also look at health risks such as foodborne illness; environmental factors; societal factors, such as drug use; and safety issues.
What Do Epidemiologists Do?
Epidemiologists have the following responsibilities:
- Identify outbreak sources. Scientists in China sourced the initial outbreak of a novel coronavirus to a market in Wuhan. Epidemiologists narrowed down the source of subsequent outbreaks in local cities and towns.
- Monitor and track disease and health behaviors. Epidemiologists are responsible for monitoring and tracking diseases and health risks in local populations. This may include monitoring seasonal flu or drug overdoses.
- Study disease. As scientists, epidemiologists study disease and its impact on a population. They may collect data, examine samples, or conduct other research as part of their job to better understand the risk to public health.
- Create public health policy. Epidemiologists advise public health officials on disease spread and other health risk factors. They help create public health policies to protect a community.
Epidemiology of COVID-19
Public health officials base the battle against COVID-19 on the data and recommendations of epidemiologists. They use various tools to slow or prevent the spread of disease. As the epidemic waxes and wanes, the tactics and epidemiology of COVID-19 change as well.
Isolation and Quarantine
At the beginning of the disease’s spread, local officials advised people to stay in their homes as much as possible. The officials closed schools and workplaces. Isolation and quarantine were widely unpopular, as they caused financial hardship for many people who lost their jobs and resulted in mental health issues for children, teens, and adults. However, it likely slowed the spread of the disease.
Masking and Social Distancing
Despite initial confusion about masking and a shortage of masks, public health scientists advised people to mask when in public. They also advised people to stay 6 feet apart at minimum and to avoid gatherings.
Data Gathering and Dissemination
As testing became more accurate, public health officials used it to track how the virus mutates and spreads. They also gathered data from hospitals on infections, deaths, and hospital capacity (number of intensive care unit beds, for example). They disseminated this data through public-facing dashboards, allowing people to identify key trends and citizens to track the disease to better understand their risk.
Vaccination Programs
As vaccines became available, public health officials rolled out vaccine programs. As different COVID-19 strains emerged, the data collected and analyzed by epidemiologists and other scientists showed waning protection and breakthrough infections. Public health officials had to also address vaccine hesitancy.
Epidemiologists Protect Public Health from the Frontlines
Epidemiologists use their training in biology and data science to understand why viruses mutate, how quickly viruses mutate, and what causes viruses to mutate. They use that knowledge to help prevent the spread of disease. They play a critical role in understanding and controlling viruses that impact communities. As part of their role as public health leaders, epidemiologists protect the health of whole populations, keeping everyone safer and allowing people to lead healthier, more productive lives.
Infographic Sources:
Centers for Disease Control and Prevention, “About COVID-19 Epidemiology”
Pfizer, “How Do Viruses Mutate and What It Means for a Vaccine?”
UnityPoint Health, “Why Viruses Mutate, Explained by an Infectious Disease Expert”