Aug 5 (Reuters) - Review website Yelp Inc (YELP.N) added new features on Thursday through which businesses can ask for proof of vaccination when customers visit, a measure it has taken as the Delta variant of the coronavirus rapidly spreads in the United States. Users will be able to filter businesses by two new attributes - "proof of vaccination required" and "all staff fully vaccinated" - when searching for local businesses and restaurants. For businesses that activate these attributes on their Yelp page, the company will add protective measures to safeguard them from reviews that criticize the enforcement of COVID-19 health and safety measures. The fast-spreading and highly-contagious Delta variant has led to several retailers reinstating their mask mandates and companies extending their work-from-home policies, just months after they lifted previous restrictions. read more As guidelines have evolved, Yelp said businesses could also activate "masks required" and "staff wears masks" attributes on their pages.