The Delta variant is causing more than 80% of new COVID-19 infections in 4 US states, including 96% of new cases in Missouri

• The Delta coronavirus variant is causing more than 80% of new COVID-19 cases in four US states.
• Kansas, Arkansas, Missouri, and Connecticut have the highest proportion of Delta cases, Scripps Research data shows.
• In Missouri, the highly infectious Delta is causing more than 96% of new cases, the data shows.

The Delta variant is ripping through parts of the US, causing more than 80% of new COVID-19 infections in four states, data shows.

The highly infectious Delta variant accounts for more than 80% of new coronavirus infections in Kansas, Arkansas, Connecticut, and Missouri, according to data compiled by Scripps Research's Outbreak.info, which is supported by the Centers for Disease Control and Prevention (CDC) and the National Institute for Allergy and Infectious Diseases.

In Missouri, more than 96% of new cases are caused by Delta, the data showed on Tuesday - the highest percentage of any US state. Outbreak.info takes data from the Global Initiative on Sharing Avian Influenza Data.

These four states have vaccination rates below 40%.

The nation's average is 47.4%, according to the CDC.

The Delta variant, first found in India, is about 60% more infectious than the Alpha variant, first found in the UK, which was formerly dominant in the US. Delta also has mutations that help it partially avoid the immune response.

Delta driving new US cases

Delta is driving new COVID-19 cases in the US, especially in areas of low vaccination.

Missouri and Arkansas, with high Delta rates, have the highest number of overall new COVID-19 infections in the US, alongside Nevada, where Delta has caused 78% of new cases, according to Covid Act Now's US COVID risk and vaccine tracker, which mostly uses CDC data.

Missouri, Arkansas, and Nevada all have below-average vaccination rates: 32% in Arkansas, 36% in Missouri, and 39% in Nevada, according to Johns Hopkins University.

The rise in Delta, coupled with low vaccination rates, socio-economic vulnerabilities and limited hospital capacity in some areas, means 12 states are now at "high" risk from COVID-19, according to Covid Act Now's tracker.

But the relationship between vaccination rates, Delta prevalence, and the number of new infections is not always straightforward.

For example, Connecticut has very low infection rates, despite a high proportion of Delta infections.

Kansas, where a below-average 39% of people are fully vaccinated, has a relatively low number of new COVID-19 cases.

And in Louisiana, where 32% of people are fully vaccinated, only 33% of new infections are caused by Delta.

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Actual figures may vary because there can be delays in uploading data, or it may not be available. Insider's Aria Bendix reported on Friday that the CDC stopped monitoring non-severe COVID-19 cases among vaccinated people in May.

Other factors could influence how fast the virus spreads. Missouri had no statewide mask mandate, for example, so local officials can set their own rules.

The spread of the virus within the state isn't uniform. Within Missouri, Iron County has 1.4 new daily cases per 1000, and Springfield has 51.2 cases per 1000.