Delta Variant Is 60 Percent More Contagious Than Original Virus and Evades Immunity

Researchers at Columbia University Mailman School of Public Health used a computer model to estimate that the Delta variant is around 60 percent more contagious than the original SARS-CoV-2 virus and can escape immunity from prior infection roughly half of the time. Compared to Delta, Beta and Gamma are less transmissible but more able to escape immunity. Compared to the original virus, Iota is more fatal to older adults.

Findings from three studies of SARS-CoV-2 variants are published on the medRxiv pre-print server ahead of publication in a peer-reviewed journal (Delta; Alpha, Beta, Gamma; Iota). [10/5/21 update: the Alpha, Beta, Gamma paper is now published in the journal Nature Communications.]

“New variants of SARS-CoV-2 have become widespread, but currently vaccines are still highly effective in preventing severe disease from these infections, so please get vaccinated if you have not done so,” says Wan Yang, PhD, assistant professor of epidemiology and lead author of the studies. “It is important that we closely monitor the spread of these variants so as to guide continued preventive measures, vaccination campaigns, and the assessment of vaccine efficacy. More fundamentally, to limit the emergence of new variants and end the COVID-19 pandemic, we need global efforts to vaccinate all populations worldwide, and continue using other public health measures until a sufficient portion of the population is protected by vaccination.”

Delta

The researchers estimate that the Delta variant (B.1.617.2) is 10 to 20 percent more transmissible than Alpha, another highly contagious variant of concern. In addition, unlike Alpha which has been shown to cause minimal immune evasion, Delta is also able to evade prior immunity in around half of individuals previously infected by the ancestral strain. These findings are in line with estimates from the U.K. that Alpha is around 1.5 times more likely to result in an infection (combining the 10-20% increase in transmissibility with <∼50% increase in susceptibility due to immune evasion for prior natural infection and, to a lesser extent, vaccination). Compared to Beta and Gamma, the Delta variant is more transmissible but less able to escape immunity. A decline in Delta cases in India beginning in early May was likely due to the implementation of public health measures and weather conditions. Monsoon season (June–September) and the winter (December–January) could see higher virus transmission. The Delta variant was first detected in December 2020 and has spread to 142 countries, as of August 10.

Alpha, Beta, and Gamma

In addition to Delta, Alpha (B.1.1.7), Beta (B.1.351), and Gamma (P.1) are the other three SARS-CoV-2 variants currently classified by the WHO as variants of concern that have spread globally. The researchers estimate that Alpha is roughly 50 percent more transmissible than the original virus but possesses only nominal ability to escape protection provided by prior infection with the original SARS-CoV-2 variant. Beta has a roughly 30 percent increase in transmissibility and 60 percent immune escape and Gamma has a roughly 40 percent increase in transmissibility and 50 percent immune escape.

Iota

Iota (B.1.526), a WHO variant of interest, is estimated to be 15-25 percent more transmissible than the original SARS-CoV-2 and slightly immune evasive (0-10%). The study, which focused on New York City, where the Iota variant emerged in November 2020, found that Iota substantially increased the infection-fatality risk compared to preexisting variants among older adults: by roughly 45 percent among 45-64-year-olds, 80 percent among 65-74-year-olds, and 60 percent among those 75 and older, during November 2020–April 2021.

Computer Model

The researchers developed computer models of COVID-19 to estimate the changes in transmissibility and immune escape for each variant, based on case and mortality data from the country where each variant emerged. Models accounted for under-detection of infection, disease seasonality, concurrent non-pharmaceutical interventions, and mass vaccination.

Additional authors can be found on individual study pages. Support for the research was provided by the National Institute of Allergy and Infectious Diseases (AI145883, AI163023), the National Science Foundation Rapid Response Research Program (DMS-2027369), and a gift from the Morris-Singer Foundation. The Iota study was also supported by the NYC Department of Health and Mental Hygiene.