COVID-19 vaccination strategy proposal: to extend the interval within doses in a situation of vaccine stockout

The vaccination campaign against COVID-19 in Catalonia is limited by the low availability of the vaccines approved (Pfizer/Bionthec, Moderna, AstraZeneca and Janssen), as well as the restrictions on the administration of the AstraZeneca vaccine in Spain. This entails that the expected impact of vaccination in the country is still very low, except in healthcare professionals and nursing homes, where hospitalizations and mortality decreased a 95%. In view of this situation, and taking into account the COVID-19 incidence and the high levels of ICU occupancy in most autonomous communities, the COVID-19 Vaccination Follow-up Group in Catalonia and the Members of the Multidisciplinary Collaborative Group for the Scientific Monitoring of COVID-19 (GCMSC), with the participation of IrsiCaixa researcher Julià Blanco, have proposed a new vaccination strategy that extends the interval between the two doses of messenger RNA (mRNA) vaccines.

The objective of this proposal is to increase vaccination of the most vulnerable population at higher risk of complications and hospitalization due to COVID-19, such as persons over 60 years of age. Lengthening the interval established in the technical data sheets of messenger RNA vaccines would allow to increase the administration of first doses and protect a higher percentage of people at risk, reducing more rapidly the number of hospitalizations and deaths. The positive impact on public health and economic dynamization are the main strengths of this intervention, which has already been adopted by some countries:

• Ireland (Pfizer vaccine: 0-4 weeks)
• Denmark, France, Germany, Italy (Pfizer and Moderna vaccines: 0-6 weeks)
• UK, Finland (Pfizer and Moderna vaccines: 0-12 weeks)
• Canada (Pfizer and Moderna vaccines: 0-16 weeks)

Taking into account that currently the 60 to 90 years age group in Catalonia is covered thanks to the administration of AstraZeneca with an interval of 12 weeks between doses, the main target population for the strategy would be those under 60 years of age.

The main parameter for defining the duration of this intervention, which is intended to be temporary, will be the percentage of the population that has received a first dose of vaccine. A reasonable scenario would be to return to the initial strategy once 50% of the population is protected with at least one dose, although this factor can be modulated depending on vaccine availability and the epidemiological situation. The parameter focuses on the experience of other countries, such as Israel and United Kingdom, where 58.1% and 51.2% of the population have already received the first dose of the vaccine and have shown a clear reduction in the incidence of hospitalizations and deaths due to COVID-19. Other countries with lower levels, such as the United States, where the percentage is 43.6%, confirm this trend.

In the report, the two groups have detailed the scientific evidence supporting the vaccination strategy they propose, as well as the mathematical models that predict the possible vaccination scenarios. On the other hand, it also takes into account possible limitations of the strategy, which is not risk-free as it implies achieving suboptimal levels of individual immunity.

Finally, the paper includes the proposals and conclusions of the research team.

Proposals of the working group

1. Maintain the interval established in the technical data sheet (21 days in the case of the Pfizer vaccine and 28 days for the Moderna vaccine) in the groups at higher risk, i.e., adults aged 60 years or older, immunocompromised persons and other pathologies defined in the vaccination strategy.
2. Lengthen the interval between the two administrations of mRNA vaccines to a maximum of 12 weeks for the rest of the population.
3. Maintain the 12-week interval with the AstraZeneca vaccine.

Conclusions

• The strategy of lengthening the interval between doses in mRNA vaccines has proven to be effective in countries where it has been implemented.
• The potential risks of this strategy are mitigable.
• The implementation of this strategy is an opportunity to influence more effectively the evolution of the pandemic and to generate highly relevant immunological-clinical-epidemiological information to optimize current vaccination campaigns and their follow-up.