The data from India and the U.K. show that Delta has emerged as the dominant variant in those countries within four to six weeks. That indicates Delta is more transmissible and infectious than the previous variants. There is emerging evidence that it can also cause more severe disease. For example, in Scotland it caused about twice as many hospitalizations than the Alpha variant, which already caused more severe illness than the original SARS-CoV-2.

“This combination of high transmissibility, high severity, and escape from vaccines makes Delta a very, very dangerous variant,” says Deepti Gurdasani, a clinical epidemiologist at Queen Mary University of London. Once Delta enters a country, it is going to spread rapidly. “It’s going to be quite hard to contain, and very likely will become the dominant variant in a matter of weeks. It could change the trajectory of the global pandemic.”

While vaccines are still effective against severe disease and hospitalization caused by the Alpha and Beta variants, they offer less protection against Delta. People who were vaccinated with one or two doses of the Pfizer vaccine produced lower levels of antibodies capable of neutralizing the Delta variant compared with the levels generated against Alpha and Beta. In the U.K., 31 percent of all confirmed Delta variant patients who needed emergency care had received at least one vaccine dose.null

Similarly, a study under review revealed that after both doses, the Pfizer vaccine showed 88 percent effectiveness against symptomatic disease caused by the Delta variant compared to 93 percent against the Alpha variant. Two doses of AstraZeneca vaccine were 66 percent effective against Alpha but only 60 percent against Delta. But with just single dose of either of the two vaccines, the vaccine effectiveness was only 51 percent against the Alpha variant compared to 33 percent against Delta. This effectiveness falls below the 50 percent efficacy threshold the FDA had set for designing safe COVID-19 vaccines; in which a vaccine should prevent at least half of the vaccinated people from getting COVID-19 symptoms.

In other studies still awaiting peer review, researchers report that Delta was responsiblefor most breakthrough infections—which occur after full vaccination—in India leading to a cluster of such cases among fully vaccinatedhealthcare workers.

There are many vaccine candidates being rolled out around the world and since there are no agreed international efficacy standards, each vaccine might offer a varying degree of protection against new variants. “We need more information about the performance of some of the more widely available vaccines in other parts of the world,” says physician and virologist Benjamin Pinsky of Stanford University School of Medicine. “I think folks need to make sure they get vaccinated. And until they are fully vaccinated, continuing with public health intervention is very important,” he says.null

A vaccine alone only slows down the progression of a contagious disease by increasing the herd immunity. Until that point, preventive measures such as social distancing and masking are proven strategies for curbing the spread of the virus.

With just 44 percent of the U.S. populationfully vaccinated, the majority of people are still vulnerable. Relaxing public health restrictions and declaring victory prematurely could provide an opportunity for the Delta variant to surge–particularly in the fall.

A study, not yet published, suggests the possibility of seasonal variations in COVID-19 incidence based on analyses from a full year of the pandemic in Europe and Israel. While the virus’s seasonal trends may not be clear yet, says Topol, we do know that when people spend more time indoors with poor ventilation and low humidity the virus spreads more rapidly.

What is happening in the U.K. could occur in many places worldwide. “We should keep social distancing after vaccination, because there will always be possibilities of breakthrough infection because vaccines can still be imperfect against emerging variants,” says Kei Sato, a virologist at the University of Tokyo, Japan, who has been studying the effect of mutations on the transmission of Delta and other emerging variants.null

“The more variants like this spread, especially in unvaccinated individuals, the more these viruses mutate and eventually pick up mutations that allow for more efficient antibody escape. This could, in theory, make the current vaccines even less effective against these variants.” Suthar cautions.

If we don’t take Delta seriously, “there will be a further wave in the U.S. We can already see the fall in cases has plateaued,” cautions Gupta. Topol agrees that if we ignore this variant “we’ll have a significant rise in cases in vulnerable areas, more hospitalization, and the pandemic here will last longer.”