The extremely transmissible delta variant of the coronavirus, which overtook all other variants in the United States just a few months ago, now represents more than 99% of cases tracked in the country, according to the data from the Centers for Disease Control and Prevention.

The variant caused caseloads to surge in Britain and India this spring and summer, and in both countries, outbreaks resurfaced after cases had seemed to be on the decline. The delta variant has been fueling outbreaks in the U.S. through this summer, its contagiousness taking advantage of the number of people who have resisted getting COVID-19 vaccinations.

The data on delta's prevalence, contained in the CDC's latest biweekly report of virus sequencing, shows it climbing from just over one-quarter of cases in mid-June to nearly total dominance in September.

"It's not unexpected, because it's more transmissible, but it is also a strong reminder that we need to have continuous vigilance," said Saskia Popescu, a public health researcher and assistant professor at George Mason University.

The CDC's COVID Data Tracker, reporting results for the two-week period ending Sept. 11, put the B.1.617.2 lineage of delta at 99.4% among variants of concern, with two other delta lineages tracked at 0.2% and 0.1%; the mu variant — first detected in January in Colombia — at 0.1%; and several other, unidentified variants at 0.2%. That data is based on thousands of sequences provided every week through the CDC's national genomic surveillance efforts, according to the agency's website.

The country recently experienced a rise in hospitalizations despite the availability of highly protective vaccines, and the delta variant has been cited as the cause.

"We're seeing more children in the hospital now because the delta variant is more readily transmissible among everybody, adults and children," Dr. Anthony Fauci, the nation's top infectious disease doctor, told The New York Times.

Popescu said the rise of delta should help Americans and health officials realize the coronavirus remains a serious public health threat.

"The biggest piece is, don't let your guard down. We need continuous surveillance, genomic sequencing, access to testing and public health interventions," Popescu said.

Vaccination and wearing masks can help, she said.

"We have transmission occurring with very limited exposure, and that means that, for example, times without a mask, when you are out and around others, become much more of a risk," Popescu said.

This article originally appeared in The New York Times.