The ongoing transmission of SARS-CoV-2 leads to the continuous generation of mutations in the virus. These mutations become concerning when they select for strains that have increased transmissibility and/or resistance to the neutralizing ability of antibodies like those induced by vaccination.
Today, the most important variant of concern is the Delta variant (also knows as the B.1.617.2 or the India variant). This variant was first recognized in India in October of 2020 and since then it has spread to many countries around the world rapidly becoming the dominant strain in circulation. For example, in the U.K. since the Delta variant was first detected in February of 2021 and, since then, it has rapidly overtaken the Alpha variant that was predominant up to that point.
Data suggests that the Delta variant is 40 to 80 percent more transmissible than pre-existing variants and, as a result, it has rapidly overtaken other variants to become the most commonly detected in many places.
Why is this variant more transmissible? The answer is not fully understood at this point, but it is likely that certain mutations may provide the virus with a greater ability to bind to the ACE2 receptor, which is where the virus attaches in order to invade host cells.
For example, the SARS-Co-V-2 Alpha (B.1.1.7) variant has a mutation (N501Y) in the spike protein that increases its affinity for the ACE2 receptor and, as a result, it is more transmissible than the original strain. The Delta (B.1.617.2) variant has several mutations in the spike protein (L452R, T478K, P681R) that not only increase binding to the ACE2 receptor but also confer increased resistance to neutralization by some of the available monoclonal antibody cocktails. In addition, these mutations cause a modest reduction in neutralization by vaccine-induced antibodies.
The impact of increased transmissibility is better understood when looking at what changes in the Ro (a mathematical term that indicates how contagious as infectious disease is). Several studies suggest that the Ro of the parent SARS-CoV-2 strain was approximately 2.5. In a 100% susceptible population the number of cases at the 10th generation would be 9,537. If the Ro increased to 2.9 then the number of cases at the 10th generation would be 42,071. This is what exponential growth is and why relatively “small” increases in the transmissibility make a huge impact.
The rapid spread of the Delta variant has prompted the World Health Organization and many counties to continue to recommend masking indoors regardless of vaccination status and reimplement some restrictions that had been previously lifted.
While the Delta variant is much more transmissible than other variants it is not yet clear if it causes more severe disease or not. Recent data from a study in Scotland suggest that the risk for hospitalization was twice that of those infected with the Alpha variant. In particular, among persons with five or more co-morbidities. Symptoms after infection with the Delta variant are similar to those of infection with other strains — but there are some reports of higher incidence of headache and sore throat early on.
How good are the currently vaccines against infection or disease with the Delta variant? In that same study both the Oxford–AstraZeneca and Pfizer–BioNTech COVID-19 vaccines were effective in reducing the risk of SARS-CoV-2 infection and COVID-19 hospitalization in people with the Delta variant, but the Oxford–AstraZeneca vaccine appeared less effective than the Pfizer–BioNTech vaccine in preventing SARS-CoV-2 infection with this variant. But it is clear that even the Pfizer-BioNTech vaccine is less effective against the Delta variant than against the Alpha variant. Two weeks after receiving the second dose of the Pfizer-BioNTech vaccine provides a 79 percent protection against infection with the Delta variant compared with 92 percent protection against the Alpha variant.
Since it was first detected in the U.S. the Delta variant has become about 20 percent of the strains in circulation in the U.S. and, with the number of cases doubling every two to three weeks, it is likely going to become the predominant variant sometime in late July. This variant appears to be spreading faster in the Midwest and the Southeast where less than 30 percent of the population is immunized. Should you be worried about the Delta variant? Only if you are not fully immunized.
Carlos del Rio, M.D., FIDSA, is vice president of the Infectious Diseases Society of America. He is a professor of medicine at Emory University School of Medicine and of global health and epidemiology at Emory’s Rollins School of Public Health, principal investigator and co-director of the Emory Center for AIDS Research, and co-principal investigator of the Emory-CDC HIV Clinical Trials Unit and Emory Vaccine and Treatment Evaluation Unit. He is also the international secretary of the National Academy of Medicine and the Chair of the PEPFAR Scientific Advisory Board.