My guess would be a combination of factors, because I don't think enough people have been vaccinated yet to entirely explain such a sharp drop. So I'd say it is a combination of vaccines, high numbers of covid infections (thus yielding some temporary immunity in many individuals), getting past the holiday gatherings, and maybe just some other dynamics with the virus. I am still expecting to see the rate of decline slow down at some point, especially with case numbers, since it's going to be a while until a lot of people can get a vaccine.