According to the World Health Organization (WHO), the Delta variant of SARS-CoV-2, the virus responsible for COVID-19 has now spread to over 100 countries worldwide.
Previously called B.1.617.2, the Delta Variant is known for its high transmissibility. Not only is the virus more contagious, it has also been linked to more severe disease.
The WHO has identified four variants of the virus, including Alpha, Beta, Gamma and Delta.
Among the four variants, Delta is believed to be the most transmissible. The Delta variant is estimated to be between 40 and 60 percent more transmissible than the Alpha variant, which was first detected in the UK.
WHO classifies a variant as a VOC when it is associated with an increase in transmissibility or detrimental change in Covid-19 epidemiology; increase in virulence; or decrease in the effectiveness of public health measures or available diagnostics, vaccines, therapeutics.
SARS-CoV-2 has about 30,000 base pairs of amino acids. An alteration in any of these bases causes a mutation that changes the shape and behaviour of the virus. The Delta variant contains multiple mutations in the spike protein. At least four mutations are important.
One of these is called L452R, first reported in Denmark in March last year. This mutation has been found more transmissible than wild-type strains and has also been associated with reduced antibody efficacy and reduced neutralisation by vaccine sera.
Evidence abound that variants with this mutation spread more quickly.
Another mutation in Delta is T478K found in around 65% of occurrences in variant B.1.1.222, first detected in Mexico last year and associated with higher infectivity.
There is also the Delta Plus variant, also known as B.1.617.2.1 or AY.1, which contains a new mutation in the spike protein the virus uses to enter human cells, called K417N.
As it’s still closely linked to Delta, it’s been called Delta Plus rather than another letter in the Greek alphabet, according to WHO’s naming system for COVID-19 variants. So far, Delta Plus has been found in relatively low numbers.
These are: increased transmissibility, stronger binding to receptors of lung cells (suggesting easier entry into human cells), and a potential reduction in response to monoclonal antibodies (which could imply that people’s immune systems may be less effective against this variant, or that they may be less responsive to lifesaving monoclonal antibody treatments).
The Delta Plus variant was found during routine screening of viral RNA sequences uploaded to GISAID, a data sharing platform set up originally to share flu viral sequences – indicating how critical ongoing surveillance is.
A Public Health England analysis showed that at least two of the vaccines are effective against Delta. The Pfizer-BioNTech vaccine was 88% effective against symptomatic disease and 96% effective against hospitalization from Delta, while Oxford-AstraZeneca (which is not an mRNA vaccine) was 60% effective against symptomatic disease and 93% effective against hospitalization. The studies tracked participants who were fully vaccinated with both recommended doses.
Moderna has also reported on studies not yet peer-reviewed that showed its vaccine to be effective against Delta and several other mutations.