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17 April 2024

Virology 101 Series: How Viruses Mutate

by Moderna
Virus Image

In the four years since the WHO declared COVID-19 a pandemic,¹ into an endemic stage, we have learned a lot about the SARS-CoV-2 virus, which causes COVID-19, with much of that understanding built upon broader industry knowledge of viruses and their epidemiology. We know today that viruses like SARS-CoV-2 continuously evolve as changes in the genetic code occur during replication of the genome.² What this means is as the virus spreads, it may change and become harder to stop.

These changes and mutations can also allow viruses to become more resistant to treatments or vaccines. In this first installment of our Virology 101 series, we’ll take a look at how both Influenza and SARS-CoV-2 viruses change and discuss how virus mutations can impact vaccine efficacy overtime and necessitate seasonal updates.

Influenza (Flu)

Flu viruses change in two main ways, antigenic drift and antigenic shift.³ Antigenic drift refers to small changes in the genes of influenza viruses that can lead to changes in the surface proteins or “antigens” of the virus, HA (hemagglutinin) and NA (neuraminidase). The changes associated with antigenic drift happen continually over time as flu viruses replicate and explain why people can get the flu multiple times over the course of their lives.³ Antigenic shift happens less frequently, and refers to an abrupt, major change in a flu A virus, resulting in new HA and/or new HA and NA proteins in flu viruses that infect humans. Antigenic shift can result in a new flu A subtype infecting people for the first time.³

Antigenic drift is one of the main reasons why the flu vaccine must be reviewed and updated each year, to keep up with the influenza virus as it changes.⁴

SARS-CoV-2 (Virus that Causes COVID-19)

SARS-CoV-2 has consistently mutated over the course of the pandemic, resulting in variants that are different from the original SARS-CoV-2 virus. A variant is a viral genetic code that may contain one or more mutations. In some cases, a group of variants with similar genetic changes, such as a lineage or group of lineages, may be designated by public health organizations as a variant of concern (VOC) or a variant of interest (VOI) due to shared attributes and characteristics that may require public health action.⁵

Research has showed that antigenic drift is highly relevant to COVID-19’s evolution as it circulates globally, pointing to a need for updating vaccines to match drifting variants, as is practiced for influenza.⁶ The CDC recommends staying up to date on all COVID-19 vaccines.⁷

Understanding viruses is paramount to understanding seasonal vaccination recommendations by health authorities. This series will pull back the curtain on virology science and demystify how viruses work.

⁵ ncov/variants/genomic-surveillance.html