Delta—one of several worrisome strains of SARS-CoV-2, the virus that causes COVID-19—is quickly becoming the dominant variant worldwide. Cases are once again rising around the globe, including in some countries that had avoided surges earlier in the pandemic. The strain is introducing new concerns about slow and uneven vaccination campaigns and vaccine hesitancy, and its spread underscores the still-long path ahead to ending the pandemic.
Delta Devastates India
The strain, also known as B.1.617.2, was first identified in India in late 2020. By the following April, it had sent the country spiraling into one of the world’s worst COVID-19 outbreaks. With the vast majority of India’s population yet to receive a vaccine dose, hospitals were pushed past their limits, and the virus killed several hundred thousand people in the weeks that followed, according to official tolls. (Most experts believe this to be a dramatic undercount.) Since then, Delta has spread to more than one hundred countries.
An Ultra-contagious Strain
Early data suggests that Delta is 30–60 percent more transmissible than the Alpha variant, which was first identified in the United Kingdom and spread globally prior to Delta. Alpha itself is thought to be 30–60 percent more transmissible than the original strain of SARS-CoV-2, meaning Delta is much more infectious than the virus that first emerged in Wuhan, China.
Is It More Deadly?
Scientists are still working to determine whether Delta is more lethal among infected individuals than earlier variants, in addition to being more transmissible; experts note that the two do not necessarily go hand in hand. Even so, a highly infectious variant carries significant risk for unvaccinated or partially vaccinated communities, as it can lead to more hospitalizations and increase strain on health-care facilities.
Pitting Vaccines Against Variants
Evidence suggests that at least several of the currently available vaccines appear to be effective against preventing death or serious illness from the Delta variant, though early research has found that they could be less effective than against earlier-identified variants. So even as cases rise around the world, many countries with relatively high proportions of vaccinated people have not experienced the spikes in COVID-19 deaths seen in those with low vaccination levels. For example, in highly vaccinated countries such as the Netherlands and United Kingdom, deaths have stayed low despite the Delta variant composing the majority of new infections.
This trend is in part due to younger people making up a larger proportion of new cases; they are at lower risk of severe infection or death. (Elderly populations often are prioritized for vaccination and make up larger percentages of those who are vaccinated.)
However, scientists warn that the vaccines’ ability to protect against the virus could lessen as significant numbers of infections continue to occur around the globe and new variants emerge that could potentially escape protection offered by available vaccines. Many experts already caution that herd immunity—when enough people in a community are protected against a virus to avoid new outbreaks—is likely impossible given lagging vaccinations and the spread of variants.
This has sparked a debate among health agencies and drugmakers about the need for booster shots that target variants of the original SARS-CoV-2 strain. But the chief of the World Health Organization sharply criticized the idea, as roughly three-quarters of the world have not yet gotten their first shot of a COVID-19 vaccine.