Q- Can I skip the second COVID-19 vaccine dose?

Preclinical trials would have shown that they didn't think there was enough immunity after one shot. So they've gone for both. Similarly, during phase three trials, there were more antibodies and T cells in the blood after two doses than there were after one. 

Pfizer and BioNTech themselves have already urged caution on the grounds that their data ends at day 21, and there is no data to demonstrate that protection after the first dose is sustained after 21 days.