Researchers have conducted a study showing that the B.1.1.7 lineage of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that emerged in the UK is now rapidly being displaced as the dominant strain in the United States by the variants of concern B.1.617.2 (Delta) and P.1 (Gamma) that emerged in India and Brazil, respectively.

A pre-print version of the research paper is available on the medRxiv* server, while the article undergoes peer review.

Since B.1.617.2 has displaced B.1.1.7 as the dominant variant in England and other countries, Alexandre Bolze and colleagues from Helix in San Mateo, California, set out to determine whether it is also displacing B.1.1.7 in the United States.

The B.1.617.2 variant was first identified in the United States on March 16th, 2021.

To examine the impact that the B.1.617.2 and P.1 variants of concern have had on the prevalence of B.1.1.7 in the United States, the researchers analyzed PCR and sequencing results of samples collected by the Helix laboratory across the country since April 2021.

The team’s analysis of polymerase chain reaction (PCR) and viral sequencing results collected from across the United States showed that B.1.1.7 is no longer responsible for the majority of new cases in the country.

The percentage of SARS-CoV-2-positive cases that were of the B.1.1.7 lineage dropped from 70% in April 2021 to 42% over a period of just 6 weeks.

Next, the researchers compared the Helix sequencing data by county with the county vaccination rates reported by the CDC.

The study showed that B.1.617.2 had a higher growth rate than P.1 and was growing faster in counties with a lower vaccination rate.

This revealed that the prevalence of B.1.617.2, which is more transmissible but less resistant to vaccination, is higher in counties with lower vaccination rates. By contrast, P.1, which is less transmissible but more resistant to vaccination, is more prevalent in counties with higher vaccination rates.

Moreover, while a study by Public Health England showed that full immunization (two doses) with the AstraZeneca or Pfizer-BioNTech vaccine remains more than 90% effective at protecting against hospitalization following infection with B.1.617.2, the efficacy following one dose is lower, compared with B.1.1.7 infection, says Bolze and colleagues.

The researchers say they expect that B.1.617.2 will soon become the dominant variant in the United States.