Scientists from the United States have recently estimated the prevalence of vaccine breakthrough cases caused by the delta variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Their findings reveal that about 58% of all new coronavirus disease 2019 (COVID-19) cases are due to the delta variant and that the prevalence of vaccine breakthrough cases associated with the variant is as high as 20%. The study is currently available on the medRxiv* preprint server while awaiting peer review.
Among newly emerged SARS-CoV-2 variants of concern, the delta variant, also called the B.1.617.2 variant, has raised concern worldwide. Soon after its first identification in India, the variant has gained rapid transmission dynamics and caused a significant rise in new COVID-19 cases in many countries globally, including Southeast Asia, Europe, and the United States. Importantly, the delta variant has been found to associate with many vaccine-breakthrough cases globally.
In the current study, scientists have estimated the prevalence of vaccine breakthrough cases caused by the delta variant in the metropolitan Houston area.
To identify cases of the delta variant, they sequenced the genomes of 3,913 SARS-CoV-2 patient samples collected between March and July 2021.
The study period covers the duration from the first identification of the delta variant in Houston until the variant became predominant in that area.
During the study period, the delta variant was found to cause about 58% of all new COVID-19 cases in the metropolitan Houston area. Of 3,913 patients studied, 213 were infected with the delta variant and 255 were identified as vaccine breakthrough cases.
Among all vaccine breakthrough cases, 84% received the BNT162b2 COVID-19 vaccine (Pfizer/BioNTech), 13% received the mRNA-1273 vaccine (Moderna), and 2% received the JNJ-78436735 vaccine (J&J/Janssen). Again, this is a reflection of the high proportion of BNT162b2 vaccinations in the U.S. health system.
The majority of patients with delta variant infection had a recent travel history to countries with highly prevalent delta cases. This indicates that multiple virus introduction events are responsible for the current outbreak of the delta variant in Houston.
Across the metropolitan area, patients with the delta variant were identified in 95 different regions. Moreover, a total of 69 subtypes of the delta variant were identified.
Importantly, patients who were infected with the same subvariant and had no apparent epidemiological connection were identified in different regions of Houston. This further indicates high infectivity and genetic diversity of the delta variant and multiple introduction events into Houston.
Although no significant correlation was observed between delta cases and age, gender, hospitalization rate, duration of hospital stay, and disease severity, Asians exhibited relatively higher susceptibility and Hispanic or Latinos showed relatively lower susceptibility to being infected with the variant.
Furthermore, patients infected with the delta variant had higher viral loads on initial diagnosis and caused a significantly higher rate of vaccine breakthrough cases compared to patients infected with other SARS-CoV-2 variants. However, in all vaccine breakthrough cases, a significantly lower viral load was noted on initial diagnosis.
The study highlights the transmission dynamics of the SARS-CoV-2 delta variant and its potency to cause vaccine breakthrough cases in the metropolitan Houston area. Based on the study findings, the delta variant is associated with 58% of all COVID-19 cases and around 20% of all vaccine breakthrough cases in Houston.
However, a significantly lower prevalence of overall vaccine breakthrough cases observed in Houston highlights that the vaccines primarily used in the United States are highly effective in preventing vaccine breakthrough cases and reducing COVID-19 severity, rate of hospitalization, and mortality.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.