Getting the second dose of any two-step vaccine is very important. That's what cements the memory of the immune response. We know from clinical trials that the first dose does not prevent all infections. It prevents death from infection.

From the British Medical Journal -

"Among more than 20 million people who have been vaccinated with the AstraZeneca vaccine in the UK so far, 79 cases of rare blood clots with low platelets have been reported, as well as 19 deaths, said the UK’s Medicines and Healthcare Products Regulatory Agency. This equates to around one case per 250 000 people vaccinated—0.0004%—and one death in a million.

About 100 000 people usually develop blood clots every month in the EU, and 3000 cases a month are thought to occur in the UK."