How contagious is it? It’s estimated that Delta caused more than twice as many infections as previous variants—in Connecticut, it was estimated to have been 80 to 90% more transmissible than the Alpha variant. In the U.S., in June 2021, after a steady decline in
COVID-19 cases and hospitalizations, the arrival of Delta coincided with a rapid reversal of that trend. In the fall of 2021, there were surges even in the most vaccinated states, prompting experts to urge people to get their booster shots.
Severity: Delta caused more severe disease than other variants in people who weren’t vaccinated. Early studies from Scotland and Canada, both cited by the CDC, suggested Delta was more likely to result in hospitalization in the unvaccinated. A report in the
Lancet this past summer found that people in England had double the hospitalization risk with Delta than they did with Alpha, the previously dominant variant in that country.
Can vaccination prevent it? All three vaccines in the U.S. were considered highly effective against severe illness, hospitalizations, and death from Delta. No vaccine is 100% effective, and Delta caused
breakthrough infections in some fully vaccinated people. Also, infected vaccinated people could spread the virus to others, although likely they were infectious for a shorter time.
Delta also prompted the CDC to recommend “
layered prevention strategies” for both the vaccinated and the unvaccinated. That means that, in addition to staying up-to-date with their vaccines, people were advised to practice such strategies as washing hands, wearing masks, and maintaining a physical distance from one another, especially when indoors in places where there was substantial or high transmission.