evidence: The study, which has not yet been peer reviewed, looked at people who had received at least one dose of the two-shot vaccines. It said the Pfizer vaccine’s effectiveness in preventing COVID-19 infection dropped to 42% in July. Overall, the study said that both the Pfizer and Moderna vaccines are highly effective in preventing hospitalization. As the virus’ delta variant has taken hold this summer, the vast majority of COVID-19 hospitalizations and deaths have occurred among people who were not vaccinated.