COVID-19 Scan for Oct 15, 2021

Vaccine effectiveness vs Delta
Impact of SARS-CoV-2 Delta variant

Vaccine effectiveness high against severe Delta-caused COVID-19

A 17-study literature review looking at vaccine effectiveness (VE) during the Delta (B1617.2) variant shows a pooled VE of 90.9% versus hospitalization with a 75.7% and 63.1% VE against symptomatic and asymptomatic infection, respectively.

The results, published yesterday in Eurosurveillance, were based on studies collected until Aug 25. Some studies looked at VE against Delta and Alpha (B117) using sequenced samples, while others looked at VE from time periods where Delta or Alpha was the dominant strain without sequencing samples. Asymptomatic infections (2 studies), symptomatic infections (9), the compound outcome of severe disease and hospitalization (3), and hospitalization alone (4) were looked at, as were VE against Alpha versus Delta (9) and waning immunity (2).

Ten studies looked at VE against infection but didn't demarcate asymptomatic versus symptomatic infections. Pooled VE was 66.9% (95% confidence interval [CI], 58.4% to 73.6%), with one study finding VE for 18- to 34-year-olds was 90%.

VE against mild outcomes was reduced by 10% to 20% when against Delta compared with Alpha but "fully maintained" against severe COVID-19, according to the researchers. Overall, VE against severe disease and hospitalization ranged from 81.5% to 100%, while VE against hospitalization ranged from 75% to 96%.

As for waning immunity, the first study found that Pfizer/BioNTech's vaccine had a 93% VE against infection (symptomatic or asymptomatic) at baseline that declined to 53% after at least 4 months of full vaccination (95% CIs, 85% to 97% and 39% to 65%, respectively). The other study looked at any COVID-19 vaccine given to US frontline workers and found a non-significant change from 85% to 73% VE after at least 5 full months of full vaccination (95% CIs, 68% to 93% and 49% to 86%, respectively).

"Current evidence shows that COVID-19 vaccines licensed in the [European Union] are moderately to highly effective in preventing SARS-CoV-2 infections with the Delta variant, while effectiveness against severe courses of COVID-19 remains high," the researchers conclude.
Oct 14 Eurosurveil study


Delta outbreak in Delhi shows immunity evasion, high transmission

Delhi's COVID-19 outbreak this past spring—the city's fourth since the pandemic began—showed the Delta variant's high transmissibility and higher ability to infect the previously infected, researchers say in a study published yesterday in Science.

The researchers used genomic and epidemiological data taken from Delhi from November 2020 to June 2021. During the 2020 outbreaks, they found that there were no variants of concern, and the Alpha variant was seen occasionally and usually in foreign travelers until January 2021. In March 2021, the Alpha variant surged to comprise about 40% of the cases, but Delta rapidly displaced its dominance in April. A Bayesian model indicated that the Delta variant was 30% to 70% more transmissible than the other COVID-19 lineages seen in Delhi thus far. And, importantly, prior COVID-19 infection provided 50% to 90% of protection against Delta compared with previous lineages.

To see how well the mathematical model held up against reality, the researchers then looked at a cohort recruited by the Council of Scientific and Industrial Research, India, which ranged from 832 to 1,012 people across three time periods. From February to July, the unvaccinated people who were positive for antibodies against COVID-19 went from 42% to 87.5%. Among 91 people who had COVID-19 before the Delta outbreak, 27.5% had increased levels of antibodies, indicating reinfection. Also, regarding breakthrough infections, a single center showed that of 24 cases, Delta was seven times more likely to be the cause.

"The concept of herd immunity is critical in ending outbreaks, but the situation in Delhi shows that infection with previous coronavirus variants will be insufficient for reaching herd immunity against Delta," said co-author Ravi Gupta, PhD, MPH, BMBCh, in a University of Cambridge press release. "The only way of ending or preventing outbreaks of Delta is either by infection with this variant or by using vaccine boosters that raise antibody levels high enough to overcome Delta’s ability to evade neutralisation."
Oct 14 Science study
Oct 14 University of Cambridge
press release

Newsletter Sign-up

Get CIDRAP news and other free newsletters.

Sign up now»


Unrestricted financial support provided by

Bentson Foundation Unorthodox Philanthropy logo and text 'Leading Underwriter'3M logoGilead 
Grant support for ASP provided by


  Become an underwriter»