Researchers at Princeton University and McGill University conducted a study that suggests that delaying the second dose of Covid-19 vaccines should reduce the case numbers in the near term. However, the longer-term case burden and potential of evolution depend on the robustness of the immune response.

Lead author Chadi Saad-Roy, a PhD candidate in Princeton’s Lewis-Sigler Institute for Integrative Genomics, said: “Several countries including the UK and Canada have stated that they will delay the second doses of Covid-19 vaccines in response to supply shortages, but also in an attempt to rapidly increase the number of people immunised.

“The original clinical trials of the vaccines, plus subsequent epidemiology, are quite optimistic regarding the efficacy of the first dose. However, we are still uncertain how the strength and duration of immunity from a single dose — or the full two-dose course or natural infection, for that matter — will persist in the longer term.”

For the study, the researchers used a simple model to project-forward the incidence of Covid-19 cases. They also tried to measure the degree of immunity of the population, under a range of vaccine-dosing regimes and assumptions related to immune responses.

The study found that one-dose strategies may reduce case numbers in the short term by more rapidly immunising a greater number of individuals. However, if immune responses after one dose are less robust, subsequent epidemic peaks may be larger.

Another important outcome associated with imperfect immune responses is the potential for viral immune escape. The theory of viral immune escape predicts that in individuals with partial immunity, moderate selection pressure combined with sufficient viral transmission could drive viral evolution.

Researchers emphasised that very low rates of vaccine administration may be associated with larger case numbers and, possibly, more elevated potential for viral adaptation.

The study was published in the journal Science .

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