The Delta variant spreads much faster, is more likely to infect the vaccinated, and could potentially trigger more severe illness in the unvaccinated compared with all other known variants, according to an internal report compiled by the Centers for Disease Control and Prevention.
The document, a slide presentation prepared by officials within the US’s health protection agency first 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 how dangerous the variant is.
Citing data from an outbreak in a county in Massachusetts, the CDC document suggested that infections in vaccinated people can produce viral loads similar to those who are unvaccinated and infected with the variant.
However, scientists acknowledge that the likelihood of vaccinated people spreading the virus, if infected, is much rarer compared with unvaccinated people.
The Delta variant is more transmissible than other viruses in the coronavirus family such as Mers and Sars, as well as Ebola and smallpox, the CDC presentation said, adding that although vaccines prevent more than 90% of severe disease, the dangerous characteristics of this variant suggest they may be relatively less effective at preventing infection or transmission.
“I think people need to understand that we’re not crying wolf here. This is serious,” Rochelle Walensky, the director of the CDC, told CNN. “It’s one of the most transmissible viruses we know about. Measles, chickenpox, this – they’re all up there.”
The report was leaked only days after the agency changed its guidance to recommend that even vaccinated people should wear masks in some indoor settings, regardless of their vaccination status, especially in areas of “substantial or high” virus transmission.
Later on Friday, the CDC released more detail on the Massachusetts outbreak among vaccinated people that informed its decision to revise mask guidance, which has prompted a backlash from conservatives across the US.
It found that around the 4 July holiday on Cape Cod, an outbreak sickened 469 people though the majority (74%) were vaccinated. Among samples that could be sequenced, the CDC found the vast majority had the Delta variant.
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.
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 underestimated).
“However, 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 also 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 90 million 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.