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The COVID-19 Delta variant: What is it and why should we worry?

In December 2020, the B.1.617.2 variant, also known as the COVID-19 Delta variant, was first documented in India (1). Originally referred to as the “India variant,” it was given the name Delta after the World Health Organization (WHO) announced a new labelling system for variants of concern on May 31, 2021 (2,3). This system was established to reduce stigmatization of countries where variants are originally detected and reported, and to streamline communication among policymakers, members of the media, and the public at large.

Given its high transmissibility, the Delta variant is currently one of the most concerning global variants of the virus in circulation, driving recent waves of infection specifically in India and the United Kingdom (1). However, experts estimate that the variant has now spread to more than 80 countries, and the Centers for Disease Control and Prevention (CDC) named it a “variant of concern,” indicating that it is more transmissible and may cause more severe disease than other COVID-19 variants in circulation (4). As of June 19, 2021, the Delta variant is now the dominant variant in the United Kingdom (UK), and CDC director Dr. Rochelle Walensky stated that it will likely become the primary variant in the United States over the next few weeks (5).

What is a variant?

As viruses spread and infect hosts in their environment, they change by means of genetic mutation (6). SARS-CoV-2 is an RNA virus and was expected to mutate as it spread over the course of the COVID-19 pandemic (7). A virus variant, or “mutant,” has a genetic sequence that differs from the reference or “parent” virus, and these slight differences in the genome impact how the virus behaves, the symptoms it causes, and/or how it is transmitted. (8) Some variants are weak and cannot persist in the environment, but others are able to infect hosts and continue to spread, like the Delta and Alpha variants (6). The Alpha (formerly “UK variant”) is currently the dominant variant in the United States.

How is the Delta variant different, and why should we worry?

The Delta variant is highly transmissible and is far more likely than other variants to impact individuals who are partially vaccinated or unvaccinated (1). Dr. Ashish Jha, the Dean of the Brown University School of Public Health, called it “the most contagious variant we’ve seen so far.” (2). Recent data support this claim: a report from Public Health England indicated that the Delta variant is between 40-50% more transmissible than the Alpha variant, which was dominant in the UK before being subverted by Delta (9). While there is ongoing research to determine whether the variant also causes more severe disease, a recent British study reported that the Delta variant is associated with higher risk of hospitalization than the Alpha variant (9).

The arrival of the Delta variant in the UK spiked infections and delayed the end of the region’s lockdown. England was slated to re-open fully on June 21, but due to rising cases, restrictions will remain in place until July 19 (1). The variant also ravaged India, whose case count remained relatively low until April and May when cases peaked at 403,000 on May 8 alone (10). While the Alpha variant is the most prevalent in the United States, experts estimate that the Delta variant will contribute to 50% of infections by early to mid-July (1, 11).

What can we do?

Doctors, public health officials, and experts in the field agree that vaccination is key to controlling the spread of the Delta variant. As of June 24, 45.9% of the United States population is fully vaccinated against COVID-19 (12). In some states, such as Vermont, Maine, Massachusetts, and Connecticut, over 60% of citizens are fully vaccinated, but in Mississippi, Alabama, Arkansas, Wyoming, and Louisiana, less than 35% are fully protected (13). More transmissible variants put unvaccinated/partially vaccinated individuals at high risk, and the Delta variant could have severe implications for these “immunologically naïve populations” (1).

While research on the Johnson & Johnson vaccine is still underway, the Pfizer/BioNTech and Moderna vaccines are highly effective against the variant after two doses (5). Pfizer/BioNTech’s vaccine is approximately 80% effective against symptomatic disease from the Delta variant in those who are fully vaccinated (2 weeks after dose 2), but the second shot is proving to be extremely important in preventing disease, as dose 1 is only 31% effective against symptomatic disease from the variant (9).

At a recent press briefing, Dr. Anthony Fauci issued a call to action. “Get vaccinated. Particularly if you’ve had your first dose, make sure you get the second. And for those who have been not vaccinated yet, please get vaccinated.” (2) Scott Gottlieb, former commissioner of the Food and Drug Administration, also emphasized the importance of taking advantage of the resources at your disposal. “We have the tools to control this and defeat it, we just need to use those tools.” (1) It’s also important for manufacturers and public health leaders to continue prioritization of equitable vaccine access worldwide. The WHO estimates that 75% of vaccine doses have been distributed to only 10 nations, putting poorer countries at high risk and heightening the importance of vaccination for those who currently have access. (14)

Given the rapid spread of the Delta variant, travelers should remain alert throughout the summer months and adhere to local restrictions and health department guidance. The CDC’s global variant map is also tracking identified virus variants and risk levels by country for international travelers (14). Travelers are likely to encounter the Delta variant this summer given its widespread prevalence, and while vaccinated individuals are highly protected, spending time around unvaccinated individuals increases the risk of infection for all. Explore the global variant map here:


  14. Link to map:

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