The Moderna vaccine appeared to be the most effective against delta after one shot, the researchers wrote, though the group said the young average age of recipients could be responsible for this finding and that a full assessment of two Moderna shots was not possible in the study.
The researchers found that while the Pfizer shot was initially much more protective than the AstraZeneca vaccine, this waned rapidly after vaccination and in four or five months matched the more stable level of protection offered by AstraZeneca.
Despite diminished protection, the researchers said that getting fully vaccinated against Covid-19 remains the best way to protect against the delta variant and that the “effectiveness of two doses remains at least as great as protection afforded by prior natural infection.”