The Delta variant could be as transmissible in vaccinated people as it is in unvaccinated people and could potentially trigger more severe illness in the unvaccinated than other coronavirus strains, according to an internal report by the Centers for Disease Control and Prevention.
The document, a slide presentation prepared by officials within the US’s health protection agency and obtained by the Washington Post, warned that the Delta variant is as infectious as chickenpox, and argues that government officials must “acknowledge the war has changed” given the severity of the strain.
The CDC document cites data that suggests vaccinated people carry viral loads similar to those who are unvaccinated and infected with the variant, although scientists acknowledge that the likelihood of vaccinated people spreading the virus, if infected, is much rarer compared with unvaccinated people. The internal CDC report, revealed by the Washington Post on Thursday, became public days after the agency changed its guidance to recommend that even vaccinated people wear masks in some indoor settings.
Rochelle Walensky, the director of the CDC, confirmed the authenticity of the report in an interview with CNN.
“I think people need to understand that we’re not crying wolf here. This is serious,” Walensky said. “It’s one of the most transmissible viruses we know about. Measles, chickenpox, this – they’re all up there.”
The Washington Post reported that the data cited in the CDC document played an important role in the agency’s decision to update its mask guidance, which has prompted a backlash from conservatives across the US. The CDC is due to publish data on Friday to support the new recommendations.
The Delta variant is as transmissible as chickenpox, the CDC presentation said, with an infected person infecting eight or nine others on average. The original strain was more comparable to the common cold, with an infected person spreading the virus to two others.
Data shared in the presentation suggested that vaccinated people who become infected with the Delta variant can shed just as much of the virus as unvaccinated people, although it emphasized that vaccines prevent more than 90% of severe disease.
“The measures we need to get this under control – they’re extreme. The measures you need are extreme,” Walensky said.
Walensky said the data in the report did not surprise her: “It was the synthesis of the data all in one place that was sobering.”
Dr Stephen Griffin, a virologist from the University of Leeds, emphasized that breakthrough infections are not common (1-2% according to the latest Public Health England summary, although with the different symptoms of Delta this may be under-estimated), and added that vaccines are still excellent at preventing severe disease and hospitalisations.
"Sin emabargo, it is concerning that when such infections do occur that the titers [concentration of an antibody] in the airway appear the same as during a non-vaccinated infection … it seems that the outbreaks mentioned are consistent with a high degree of infectious virus being produced,” Griffin told the Guardian.
“We must remember though, going forward, that we will see a greater proportion of [vaccinated people]becoming infected as the coverage increases within the population, but the difference here is that Delta certainly appears to do this more often and with more potential for symptomatic infection compared to other variants.”
The CDC document highlights “communication challenges” and says the agency must change its public messaging to emphasize vaccination as the best defense against the Delta variant.
The US is averaging almost 62,000 new Covid-19 infections a day, and the vast majority of those hospitalised and dying have not been vaccinated. Nationwide 49.8% of Americans are fully vaccinated, according to the CDC.
On Thursday Joe Biden described the surge as a “pandemic of the unvaccinated”, and said about 90m Americans who are eligible for a shot have not yet got one.
“Masking is one defense against the spread of Covid-19 but make no mistake: vaccines are the best defence against you getting severely ill from Covid-19. The very best defence,” the president said.