Delta Variant Dominance in the U.S.
A big-picture look at the Delta variant and what we can learn from early data to prevent another wave.
The Big Picture
The week of June 14th, data from genomic sequencing suggests that the Delta variant (B.1.617.2 first identified in India) is roughly doubling every week and we have reached the time period when it is estimated to become the dominant strain in the United States. While it is a Variant of Concern (VOC) that fits all three criteria of transmissibility, severity, and immune escape, there is good news that fully vaccinated individuals retain high levels of immune protection although slightly lower protection when compared to previous strains of the virus. Additionally, cases in the UK show greatly diminished protection for those partially vaccinated and high levels of cases and hospitalizations for those who are unvaccinated. While there is optimism in light of record low case numbers not seen since March 2020 in the United States, fully vaccinated rates vary greatly across the country, with some counties as low as 10% indicating that many communities will be vulnerable to outbreaks. Recent trends show slowing vaccination rates as the country experiences challenges of access, trust, and misinformation. These challenges in combination with mass reopenings suggest that we will follow a similar path to the UK of increased cases in the coming weeks and should quickly adjust our strategy to suppress cases through increased communication, vaccination initiatives, and protection protocols.
Caveats: The prevalence numbers are only estimates due to a very small percentage of COVID tests being sequenced across the country and lag in reporting data. Additionally, studies on severity cited in this article are early and in large part preliminary and more studies are ongoing to provide greater details on transmissibility, immune escape, and severity.
Delta Variant Dominance ETA
The United States has had weekly increases of Delta variant cases ranging between 75% to 100%. The last samples between June 4th and June 11th more than doubled with an increase of 135% over the previous week. If we assume a conservative increase for this past week at 75%, then that puts us at 49% prevalence as of June 18th. Based on the range of weekly growth over the last several weeks it is likely we have just passed the dominance threshold for the Delta variant.
Good News, Bad News
We have the lowest daily case numbers of COVID-19 not seen since the beginning of the pandemic in the United States. While this a reason to celebrate, state reopenings and the dropping of many protection protocols give a false sense that we are done with this pandemic. The UK’s similar decrease in cases and vaccination rates gives us a helpful preview of what we can expect in the U.S.
Vaccination Rates
The official date the Delta variant became dominant in the UK, May 18th, is the same date their cases started to consistently rise. At that point, 54.48% of their population had received one dose and 30.74% of their population were fully vaccinated. In comparison, at the estimated time that the United States may have reached Delta dominance, roughly June 17th, we would roughly have had 52.59% with one dose and 44.18% fully vaccinated.
Vaccination rates vary greatly across the country, with fully vaccinated ranging from as high as 70% in some counties to as low as 10% in other counties.
Recent data from the CDC shows that 40% of counties are below 30% fully vaccinated giving us an idea of the level of vulnerability across the country.
Variant of Concern
To be a Variant of Concern, a variant has to fit one of the three categories of either increased transmissibility, severity, or immune escape. The Delta variant shows an increase in all three of these categories.
Transmissibility
There is evidence that the Delta variant is the most transmissible variant to date. One study shows it is 64% more transmissible than the Alpha (B.1.1.7) variant which was estimated in a research article published in April to be anywhere between 43% and 90% more transmissible than the original wild type and another study finding it was 50%-100% more transmissible.
On June 18th, a month after the UK reached 50% prevalence of the Delta variant, Public Health England shared in a press release, “The most recent data show 99% of sequenced and genotyped cases across the country are the Delta variant.”
Highlighting Delta’s fitness over other variants, the Chief Scientist of the World Health Organization, Soumya Swaminathan, said at a recent press briefing, “The Delta variant is well on its way to becoming the dominant variant globally because of its significant increase in transmissibility.”
Immune Escape
The key takeaway here is that vaccines are still effective against the Delta variant. Although there is an increase in this variant’s ability to evade immunity being fully vaccinated offers a high level of protection, being partially vaccinated offers much less, and being unvaccinated is the vast majority of cases and hospitalizations.
A recent study from Public Health England gives more details on how effective the vaccines are against the Delta variant (B.1.617.2):
- the Pfizer-BioNTech vaccine was 88% effective against symptomatic disease from the B.1.617.2 variant 2 weeks after the second dose, compared to 93% effectiveness against the B.1.1.7 variant
- 2 doses of the AstraZeneca vaccine were 60% effective against symptomatic disease from the B.1.617.2 variant compared to 66% effectiveness against the B.1.1.7 variant
- both vaccines were 33% effective against symptomatic disease from B.1.617.2, 3 weeks after the first dose compared to around 50% effectiveness against the B.1.1.7 variant
While this is great news for the United States, there is more data needed to check the change of effectiveness in all the vaccines being distributed across the world. (J&J is an adenovirus vaccine similar to AstraZeneca and Moderna is an mRNA vaccine similar to Pfizer-BioNTech. I will update this post as soon as J&J and Moderna data are available).
Severity
Early data shows that severity for the Delta variant is estimated to be more than 3.3x that of the wild-type coronavirus that started the pandemic. These numbers can be obtained by taking Alpha’s severity increase of 64% above that of the wild type coronavirus (UK study) and taking the average of Scotland’s and England’s recent severity increase of Delta cases over that of Alpha cases (Public Health England report), which is a formula of 164%*((2.26+1.85)/2), respectively.
Another study out of Scotland provides supporting data of this increase of hospital admissions when comparing the Delta variant to the Alpha variant sharing that “Risk of COVID-19 hospital admission was approximately doubled in those with the Delta VOC when compared to the Alpha VOC.”
What’s Next?
The Delta variant is here and while the U.S. has a higher rate of fully vaccinated individuals than the UK did at the point of the Delta variant becoming dominant, the disparity amongst counties with low vaccination numbers shows the underlying vulnerabilities for spread that can lead to many outbreaks throughout the U.S. Focusing on getting ahead of misinformation by communicating early and clearly to address the impacts of the Delta variant on vaccines, spread, and illness will be helpful in helping people understand the benefits of vaccines and other protection protocols helpful in preventing spread. Additionally, there are tools that can make a difference in controlling outbreaks like increasing genomic testing, implementing quality mask recommendations, increasing ventilation, and testing the vaccinated after exposure to improve data on vaccine effectiveness against new variants and increase data on secondary infections. We have all the tools necessary to prevent another wave, let’s use them.
Follow-up publication: Part 2: Delta Variant Dominance in the U.S. and Unvaccinated Vulnerability
