How similar is SARS-CoV-2 to Influenza: A wolf in sheep’s clothing
In this post, we discuss how similar SARS-CoV-2 is to Influenza. Are these two viruses actually that different? What’s the science behind their differences? Read on for more.
Since the outbreak of SARS-CoV-2 in China in late 2019—the new headline grabbing respiratory pathogen—there has been a lot of conversation regarding how influenza (commonly called the flu) compares to SARS-CoV-2. It’s important to compare and contrast the two viruses, especially considering many consider them nearly identical. This is not true. While there are some similarities, we must understand that both viruses are inherently distinct. Assumptions of their similarity can be dangerous. So, let’s begin with why they seem similar:
How they present: Both Influenza and SARS-CoV-2 are viruses that primarily cause disease in humans in the respiratory tract, often resulting in common symptoms. These include fever, cough, fatigue and can also include stuffy or runny nose, sore throat and, in certain cases, diarrhea.
Key point: You can be infected by either virus and present similar symptoms.
Image credit: https://blog.watsonconsoles.com/3-steps-help-psaps-beat-cold-and-flu-call-outs
2. How they spread: Both viruses spread through respiratory droplets, small droplets released when we sneeze or cough. These can prevail on surfaces where other people can come in contact and get infected. An effective way to stop the spread of both is to socially distance and stay at home if sick.
Key point: Both viruses spread through droplets released upon sneezing and coughing; isolating yourself is the best way to limit spread.
3. What they are made of: Both of these viruses contain genetic material that is composed of RNA (ribonucleic acid) and can change themselves rapidly as they multiply. Over time, these changes may allow them to gain new abilities like being highly infectious, infecting other animals, etc. Both viruses are similar in structure by having an envelope around the virus made of lipids (essentially fat!). This makes strategies like using detergents (wash your hands!) or alcohol effective in rendering the virus non-infectious as they can disrupt that envelope around the virus that is necessary for infectivity.
Key points: Both viruses are made up of RNA and can acquire changes in their genetic material over time as they multiply. Using good hygiene (hand washing) can help inactivate the viruses.
Image: https://www.cdc.gov/flu/prevent/preventing.html
However, despite these general similarities, these viruses differ a great deal! Just because both viruses infect similarly or can feel the same symptomatically for some patients does not mean they are the same thing.
Let’s look at some key differences between influenza and SARS-CoV-2.
What they are: You might be wondering, if they are both composed of similar genetic building material (RNA) as mentioned above then what makes them different? The simple answer is the content and organization of their genetic building material. Influenza belongs to the Orthomyxoviridae family of viruses that are distinct from the Coronaviridae family for SARS-CoV-2. Viruses can be classified based on many features including genetic material. Since both viruses are from distinct families, this means that they are genetically distinct and do not share a common ancestor.
Another interesting fact about influenza is that it has a distinct, segmented genome—this virus contains separate pieces of genome that can mix and match between different strains of the virus and result in new versions of influenza. On the other hand, coronaviruses contain a single piece of their genome tightly wrapped inside the virus particle. They can both accumulate changes in their genetic material as they multiply, but SARS-CoV-2 does not have the flexibility of a segmented genome like influenza.
Key point: They both belong to different families of virus, differ in their genetic material and composition. Influenza may change more over time than SARS-CoV-2.
Image made by Praneet Sanhu
2. How novel each virus is: Influenza has been responsible for many outbreaks and results in numerous deaths every year. However, we know much more about flu than SARS-CoV-2. For flu, we have been able to track how many people get infected, learn how the virus infects, and investigate the biology of the virus. Based on this, we have antiviral drugs that are effective for influenza as well as an annual vaccine which is manufactured based on what virus strains circulate and how much they might have changed. Using all of this knowledge, antivirals and vaccines allows us some degree of control. In contrast, SARS-CoV-2 is novel. Because the virus is new, we have no pre-existing immunity and the virus is ‘foreign’ to our bodies. Additionally, doctors, scientists and epidemiologists around the world are trying to understand how this virus behaves and figure out ways we can protect ourselves, but until then we have no tailor-made drugs or vaccines.
A great example of an emerging pathogen was the deadly 1918 flu pandemic. We lacked antivirals and a vaccine and the virus spread efficiently amongst people, infecting more than 1/3 of the world’s population. Since then, we have become better at combatting the influenza virus. Despite regularly recurring influenza infections every season, we are able to control the spread of the virus through antivirals and vaccinations in addition to some pre-existing immunity. Because most influenza strains have been circulating within the human population for many years so people have some level of pre-exisiting immunity to these strains. Importantly, the 1918 pandemic flu was a type of flu for which there was very little pre-existing immunity as well. The combination of therapies, vaccination and immunity make us better equipped to deal with most influenza outbreaks today. For SARS-CoV-2, we have none of these things. We do not yet have any antivirals or vaccines to prevent the spread of the virus and we are still learning, on a daily basis, about the virus biology. Thus, the novelty of the virus present a special set of challenges: a lack of treatments and a lack of natural and vaccine-based immunity.
Key point: Because SARS-CoV-2 is so new, we so far have no vaccines or antivirals as well as no immunity, making it especially difficult to control.
3. How infectious and deadly they are: Based on the total number of infections and deaths due to infections, we can estimate the mortality rate for these viruses. Typical seasonal influenza is calculated to be about 0.02 - 0.1%, i.e., between 2-10 in 10,000 people infected with influenza die from the infection. For SARS-CoV-2 the case fatality rate is estimated between 1-10%, although the best estimates by the WHO, Johns Hopkins, and the Lancet put it, more specifically, between 1.5-3.4% overall. Hence, the death rate occurring from SARS-CoV-2 is likely to be 10-100 times greater than that of influenza. While many factors contribute to these numbers, these estimated percentages tell us that the risk of succumbing to disease is higher for SARS-CoV-2 than influenza. These percentages can rise in countries where the healthcare system becomes overwhelmed, reaching up to an 11% mortality rate. Further, approximately 20-30% of patients who develop COVID-19 (the disease due to SARS-CoV-2) require hospitalization and 5-10% require ICU admissions in the US. This puts a significant burden on the healthcare system as COVID-19 cases rise. Additionally, the average incubation period (time period between first exposure to virus to development of symptoms) is shorter for influenza, ranging about 1 to 4 days, while SARS-CoV-2 has a range of 1 to 14 days. Thus, due to the longer incubation period of SARS-CoV-2, people can spread the virus for a longer period of time and even have a potential window of infectiousness before being symptomatic.
Key point: The case fatality rate due to SARS-CoV-2 is at least ten times higher than seasonal influenza with higher hospitalization rates and a larger window for transmission.
4. Who is in the risk group: In the case of influenza, the vulnerable groups of population are the elderly, young children and people with pre-existing medical conditions. This risk group also extends to pregnant women for influenza. For SARS-CoV-2, the risk groups include elderly people (especially over 65 years of age), people with weakened immune systems and underlying health complications like lung disease, heart disease, diabetes, etc. Children do not seem to exhibit vulnerability to SARS-CoV-2 like they do for influenza. However, this does not mean that they cannot be infected; it means SARS-CoV-2 infections do not causes significant morbidity in children. Currently, there is little data to suggest that pregnant women have increased risk of SARS-CoV-2 infections, but that is still being investigated (stay updated for a post in press about this very topic). However, it is important to mention that, although the percentage is low, young and healthy people can still get infected, require medical attention and even die due to COVID-19.
Key point: Older people and people with underlying medical conditions are at higher risk for SARS-CoV-2, whereas this extends to children for influenza.
Image credit: Vox media
In summary, influenza and SARS-CoV-2 may look alike due to their respiratory nature of infection, but they are not the same. It is important we recognize their differences because those differences influence the decisions we make in fighting against each of them. So, keep the spread of these viruses in check, practice social distancing and wash your hands!
Praneet Sandhu
Praneet is currently a PhD student at Penn State College of Medicine in Hershey, PA. A virologist by chance, Praneet studies how cellular proteins are manipulated by human cytomegalovirus, a herpesvirus, for its own benefit. She is very interested in learning how viruses have figured out multiple ways to overcome host defenses. Outside of science, she likes to read about evolution and world history, and bake with her husband.