Looks like actual cases will make up a material portion of herd immunity. Columbia reports that up to 1/3 of Americans already had contracted COVID by the end of 2020:
https://www.publichealth.columbia.edu/public-health-now/news...
https://www.publichealth.columbia.edu/public-health-now/news...
Here’s why herd immunity from Covid is ‘mythical’ with the delta variant https://www.cnbc.com/2021/08/12/herd-immunity-is-mythical-wi...
The developer of the AstraZeneca shot says the Delta variant has made herd immunity impossible because vaccinated people can still transmit the virus https://www.businessinsider.com/delta-variant-made-herd-immu...
Natural immunity is great, but most people clearly don't have and won't get natural immunity. Having had COVID-19 already also does not make you later immune, to any variant. You can get it again and spread it again. https://health.clevelandclinic.org/can-you-get-covid-19-more...