That's not what the New England Journal of Medicine says[1]:
> Only modest differences in vaccine effectiveness were noted with the delta variant as compared with the alpha variant after the receipt of two vaccine doses.
> Unfortunately, the vaccine’s beneficial effect on Delta transmission waned to almost negligible levels over time. In people infected 2 weeks after receiving the vaccine developed by the University of Oxford and AstraZeneca, both in the UK, the chance that an unvaccinated close contact would test positive was 57%, but 3 months later, that chance rose to 67%. The latter figure is on par with the likelihood that an unvaccinated person will spread the virus.
A reduction was also observed in people vaccinated with the jab made by US company Pfizer and German firm BioNTech. The risk of spreading the Delta infection soon after vaccination with that jab was 42%, but increased to 58% with time.
does not mean "effectiveness at preventing the spread of the virus", it means "effectiveness at preventing severe illness and death".