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US air travel hits highest level since March 2020


The Delta variant, or the B.1.617.2 first identified in India, is one of the variants overtaking the Alpha, or B.1.1.7 variant first identified in the United Kingdom, as the dominant variant in the United States, and it’s happening faster in counties with lower vaccination rates, according to a new study by scientists at Helix, scheduled to be published as a preprint in the coming days.

Scientists at Helix analyzed nearly 20,000 Covid-19 tests collected since April 2021 and just under 250,000 Covid-19 sequence results of samples Helix collected since January 2021. 

What the research showed:

  • They found that the percentage of positive cases of the B.1.1.7 variant dropped from 70% in April 2021 to 42% six weeks later.
  • Their results show that “the variant of concern B.1.1.7 is rapidly being displaced in the United States,” they said, and most of this displacement can be attributed to the Delta variant and the Gamma variant, also known as P.1 and first identified in Brazil. “In the United States, this analysis showed that the growth rate of B.1.617.2 was faster than P.1,” said the research.
  • However, growth rates of the two variants differed by the county vaccination rate.The samples the study looked at came from 747 counties. The sequence data from the counties was compared to county vaccination rates that came from the US Centers for Disease Control and Prevention.

“The growth curve for B.1.617.2, which is more transmissible but against which vaccines are highly effective, shows faster growth in counties with lower vaccination rates,” said the study. “In contrast, P.1, which is less transmissible but against which vaccines have somewhat less efficacy, has a higher prevalence in counties with higher vaccination rates.”

More on the study: They defined a county with a lower vaccination rate as one which had less than 28.5% of the population completely vaccinated on May 1, the others were considered counties with a higher vaccination rate.

An important limitation is noted by the researchers: the relatively small number of positives that have been analyzed in the last two months, partly because of the lower numbers of cases in the US and the decrease in test positivity rate. The data is not homogenous across the US and the samples “do not proportionally represent the different areas of the United states by population”, they say, which is another limitation.


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