SARS-CoV-2, the virus causing COVID-19, constantly changes as it spreads among people. These changes lead to the emergence of new versions of the virus, known as variants. While viruses naturally evolve over time, these genetic modifications can significantly influence how the virus behaves.
What are Variants and How They Emerge
Viruses, including SARS-CoV-2, reproduce by making copies of their genetic material inside host cells. This copying process is not always perfect, and small errors, called mutations, can occur in the virus’s genetic code. Coronaviruses, which use RNA to store their genetic information, are particularly prone to these copying mistakes.
Most of these mutations are either harmless or detrimental to the virus. However, a small number can provide an advantage. When a mutation changes how a virus behaves in a population, it is identified as a variant. These advantageous changes can help the variant spread more easily or evade the body’s immune response.
Why Variants Matter
Variants of SARS-CoV-2 are a concern because they can alter key characteristics of the virus, impacting public health in several ways. One significant concern is increased transmissibility, meaning a variant can spread more easily from person to person. For example, the Alpha variant was about 86% more contagious per week than the original virus, and the Delta variant was estimated to be 216% more contagious than Alpha.
Variants can also affect disease severity, influencing how sick people become after infection. The Delta variant, for instance, was linked to increased severity and higher rates of critical illness and mortality compared to Alpha and Omicron variants.
Furthermore, variants can reduce the effectiveness of existing tools used to combat the virus, such as vaccines, treatments, and diagnostic tests. Mutations in the virus’s spike protein, which it uses to attach to host cells, can allow it to evade protective antibodies generated by prior infection or vaccination. This can lead to breakthrough infections, even in vaccinated individuals, though vaccines generally still help prevent severe illness.
Tracking and Naming Variants
Scientists monitor the emergence and spread of coronavirus variants through a process called genomic sequencing. This involves analyzing the complete genetic code of virus samples to identify new mutations and track their prevalence. Real-time sharing of these genome sequences has been instrumental in understanding the virus’s evolution and transmission dynamics globally.
To communicate about variants, several naming conventions are used. The World Health Organization (WHO) assigns simple, easy-to-remember labels using letters of the Greek alphabet for variants of concern (VOCs) and variants of interest (VOIs). This system was introduced to avoid stigmatizing countries where variants were first identified. Examples include Alpha, Delta, and Omicron.
Another scientific naming system is Pango lineages. This hierarchical system uses alphabetical prefixes and numerical suffixes (e.g., B.1.1.7) to indicate the evolutionary relationship and ancestry of a variant. The Pango nomenclature is widely used by researchers and public health agencies to track the transmission and spread of SARS-CoV-2, including variants of concern.
Key Variants and Their Characteristics
The Alpha variant (Pango lineage B.1.1.7) was first identified in the United Kingdom in late 2020. It quickly became prevalent in Europe and showed increased transmissibility compared to earlier strains, leading to a rise in hospitalizations.
The Delta variant (Pango lineage B.1.617.2) emerged in India in early 2021 and rapidly became the dominant global strain until late 2021. This variant was highly contagious and associated with more severe disease, particularly in unvaccinated individuals.
The Omicron variant (Pango lineage B.1.1.529) was first detected in southern Africa in November 2021 and quickly became the main variant globally, replacing Delta. Omicron lineages, such as BA.1.1 and BA.2, showed a higher number of mutations compared to Alpha and Delta variants. While highly transmissible, Omicron generally caused less severe disease compared to previous variants like Delta.