Delaying second doses of COVID-19 vaccines has benefits, but longer-term results rely on robust immunity Delaying second doses of COVID-19 vaccines should reduce case numbers in the near term. But the longer-term case burden and the potential for evolution of viral "escape" from immunity will depend on the robustness of immune responses generated by natural infections and one or two vaccine doses, according to a Princeton University and McGill University study published March 9 in the journal Science. Several countries including the United Kingdom and Canada have stated that they will delay second doses of COVID-19 vaccines in response to supply shortages, but also in an attempt to rapidly increase the number of people immunized."