SARS-CoV-2 is a novel virus, which means that we had no available treatments or immunity to the pathogen when it emerged in late 2019. However, only 131 subtypes have been detected in nature, to date. There are 18 possible HA subtypes and 11 NA subtypes, which means 198 combinations are possible. Influenza A virus is further divided into subtypes based on its HA and NA surface proteins. The 2 most important strains of influenza virus, when it comes to human disease, are influenza A and influenza B, which both cause annual seasonal flu outbreaks. Previously circulating VOCs such as Alpha, Beta and Gamma are either no longer detected or are circulating at such low levels that they do not pose a significant, imminent threat to public health. 2022, Omicron had outcompeted Delta, accounting for >98% of all viral sequences shared on GISAID. 2021, the highly transmissible Delta variant became the dominant circulating variant in the U.S., and by Feb. SARS-CoV-2 variants are classified as variants under monitoring (VUM) variants of interest (VOI) and variants of concern (VOC). NA cleaves sialic acid from the cell surface, which releases HA and allows progeny viruses to exit infected cells and continue spreading.Īnother important difference between SARS-CoV-2 and influenza is that there are multiple variants of SARS-CoV-2 and 4 different strains (A, B, C and D), and many different subtypes, of influenza virus. However, as long as HA remains bound to sialic acid on cell surfaces, newly synthesized virus particles are unable to exit the infected cells. Once inside, influenza virus also releases its RNA to be copied and synthesized into new virus particles. HA preferentially binds to sialic acid on the surface of respiratory epithelial cells, and mediates entry of the virus to host cells. The host cell receptor for influenza viruses is sialic acid, a sugar chain that is fairly ubiquitous and attached to surface lipids and proteins of most host cells, as well as soluble proteins. Influenza viruses rely on the collaborative functions of 2 viral surface proteins, haemagglutinin (HA) and neuraminidase (NA) to enter and exit host cells. Newly synthesized virus particles are then released to infect additional host cells. Upon binding, SARS-CoV-2 injects its RNA into the infected cell and uses host cell machinery to replicate its genome. S proteins bind to the host cell receptor, angiotensin-converting enzyme 2 (ACE2), which regulates blood pressure and fluid-salt balances and is expressed by multiple organ systems throughout the body, including the lungs, heart, kidneys, liver, intestines, brain and adipose tissues. SARS-CoV-2 is covered in spike (S) proteins that facilitate invasion of host cells. SARS-CoV-2 has single-stranded, non-segmented, positive-sense, viral RNA.īoth viruses possess distinguishing surface proteins that serve as important virulence factors for infection. Influenza virus is comprised of 8 single-stranded, negative-sense, viral RNA segments. However, the genomes of these 2 viruses differ in polarity and segmentation. Source: American Society for MicrobiologyĬoronaviruses and influenza viruses are both enveloped, single-stranded RNA viruses, and both are encapsidated by nucleoprotein.
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