Those who had mild or asymptomatic cases of the April 2020 variant are susceptible to reinfection by the South Africa and Brazil variants due to changes in the S-protein structure rendering the antibodies less effective. The levels of antibodies in those who had mild or asymptomatic cases are typically insufficient to contain the infection with those variants. This has been observed in both countries to a significant degree.
"You might want to check your calendar."
The first recipients of vaccine doses in the Phase 1 clinical trials got their shots March 16, 2020. My calendar is just fine.