Individuals infected with the Delta variant of SARS-CoV-2 are experiencing more severe illness compared to earlier strains. This variant spreads as efficiently as chickenpox, per a forthcoming CDC analysis detailed in an internal document.
Detected in 132 countries, Delta now drives most new infections worldwide. Experts knew it was more virulent, but new data underscores its exceptional transmissibility, as revealed in a Centers for Disease Control and Prevention (CDC) document obtained by The Washington Post on July 29.
A disease's reproduction number (R0) estimates how many people one infected person can infect. The original SARS-CoV-2 had an R0 of about 2-3. CDC data shows Delta is up to four times more contagious, with an R0 around 8 or higher.
This surpasses MERS, SARS, Ebola, smallpox, and seasonal flu, matching chickenpox's infectivity.
"I'm much more concerned after reading it," wrote Robert Wachter, MD, chair of medicine at UC San Francisco, in an email to The Washington Post. CDC Director Rochelle Walensky told CNN, "This is serious. It's one of the most transmissible viruses we know of."
Delta can evade vaccine protection to some extent, allowing transmission regardless of vaccination status. "Vaccinated people with no symptoms infected and shed virus at levels similar to the unvaccinated," noted Walter Orenstein, MD, from Emory Vaccine Center.
"Vaccinated individuals likely contribute significantly to Delta transmission," said epidemiologist Jeffrey Shaman of Columbia University. "Vaccination now primarily protects against serious illness." Herd immunity grows harder to achieve amid Delta's spread.
Vaccines remain vital: Vaccinated people face less than one-tenth the risk of severe disease or death compared to the unvaccinated. Infection risk drops by a factor of three for the vaccinated versus unvaccinated.
