The Delta variant (B.1.617.2) was a significant form of the SARS-CoV-2 virus. First identified in India on October 5, 2020, it rapidly gained global attention. The World Health Organization (WHO) designated the Delta variant as a “variant of concern” due to its rapid spread and potential impact. By mid-2021, the Delta variant became the dominant strain worldwide, displacing other circulating lineages.
Key Characteristics
The Delta variant exhibited defining features that made it particularly transmissible and concerning. It was estimated to be 40% to 80% more contagious than the Alpha variant, which itself was more transmissible than the original SARS-CoV-2 strain. This heightened transmissibility allowed it to spread quickly between individuals, leading to rapid increases in case numbers.
One reason for its increased contagiousness was the presence of specific genetic mutations, particularly in the virus’s spike protein. Mutations like T478K, P681R, and L452R affected the spike protein, which the virus uses to attach to and enter human cells. These changes enhanced the virus’s ability to bind more efficiently to host cells and replicate faster.
Individuals infected with the Delta variant often carried a significantly higher viral load compared to those infected with earlier strains. Research indicated that Delta variant infections could produce up to 1,000 times more viral material in the respiratory tract than the original virus. This higher viral load meant infected individuals were more likely to transmit the virus when coughing, sneezing, or talking. The basic reproductive number (R0) for the Delta variant was estimated to be around 5.08 to 6.4, substantially higher than the R0 of approximately 2.79 for the ancestral strain.
Clinical Presentation and Severity
The symptoms associated with the Delta variant were largely similar to those of previous SARS-CoV-2 strains but with some notable differences in prevalence. Common symptoms included fever, headache, sore throat, and a runny or stuffy nose. However, a persistent cough and the loss of taste or smell, which were prominent symptoms with earlier variants, became less common with Delta infections.
Studies suggested that Delta variant infections could lead to more severe disease outcomes, including a higher risk of hospitalization and the need for oxygen, especially in unvaccinated individuals. For instance, some research indicated a significantly increased risk of hospitalization from Delta compared to Alpha. This heightened severity contributed to increased burdens on healthcare systems.
The time from exposure to symptom onset for the Delta variant was typically around four days, which was a bit faster than previous variants. While the variant could cause more severe illness, particularly for unvaccinated people, the overall range of COVID-19 symptoms remained consistent.
Impact on Immunity and Prevention
The Delta variant posed challenges to existing immunity, whether acquired through prior infection or vaccination. While prior infection with SARS-CoV-2 offered protection against reinfection, its effectiveness against the Delta variant varied. One study found that prior infection provided approximately 85.4% protection against reinfection with Delta, with immunity lasting at least 13 months. However, some evidence suggested that Delta was less sensitive to antibodies from naturally immunized individuals, emphasizing the benefit of vaccination even for those with previous infections.
Vaccines available during the Delta variant’s prevalence demonstrated good effectiveness, particularly against severe disease, hospitalization, and death. Two doses of mRNA vaccines (Pfizer or Moderna) and the AstraZeneca vaccine showed high effectiveness, often exceeding 90%, in preventing death from Delta infections. While vaccinated individuals could still experience breakthrough infections, these were often milder and less likely to result in severe outcomes or hospitalization.
Public health measures remained important in mitigating the spread of the Delta variant. Recommendations included continued masking, particularly in public indoor places, and maintaining social distancing. These measures, combined with increased vaccination rates, were considered effective means to control the variant’s spread.