The virus was around almost a whole yr before the release of vaccines. So scientists have been studying "natural immunity" for awhile. And before vaccines were available, people with prior infections were getting sick a second time (some severely). Stuff on waning:
SARS-CoV-2 anti-nucleocapsid antibodies wane within months and fall faster in younger adults and those without symptoms. However, anti-spike IgG remains stably detected. Ongoing longitudinal studies are required to track the long-term duration of antibody levels and their association with...
pubmed.ncbi.nlm.nih.gov
Illness severity in COVID-19 correlates with specificity of serological responses, but antibody levels decrease in most patients.
www.science.org
This is an interesting read, and I think it still holds true that average natural infection and vaccines both offer a similar protection for 6 - 8 months.
The US CDC estimates that SARS-CoV-2 has infected more than 100 million Americans, and evidence is mounting that natural immunity is at least as protective as vaccination. Yet public health leadership says everyone needs the vaccine. Jennifer Block investigates When the vaccine rollout began in...
www.bmj.com
The stuff i'm referencing was pretty recent.
The US definitely lags other countries in the scope of recognition though. I suspect one reason is because the US has had high availability of the vaccines compared to other countries. Other countries were forced to ponder the protection of prior infections with availability of shots.