After a year, SARS-CoV-2 has taken the world by storm, affecting up to 81 million and killing more than 1.8 million people world wide.1 Treatments have proven to be of little or of no efficacy, and there are progressively increasing number of patients with late sequelae.2 At this point, with a vaccine already in place, there is a ray of hope for slowing the advance of this disease.
Spain has been particularly hit by COVID-19, with nearly 2 million of infected as for 2 January 2021.1 Since 29 December 2020, the vaccine against SARS-CoV-2 Cominarty® (Pfizer-BioNTech) has been implemented in our country following a strategy that prioritizes population groups to be vaccinated, including the protection of the most vulnerable people.3 To achieve a 95% efficacy the first dose should be followed by a second 21 days later.4
But with more than 50,000 deaths in Spain behind us, and in the context of a high and increasing incidence of COVID19 in the last weeks, it might be placed a high priority on promoting rapid, high levels of vaccine uptake. This can be more easily and quickly achieved if the current scheme is modified, as already recommended in other countries, as UK.5 While the trial data shows that the vaccine conferred immunity to 52% of the participants three weeks after the first dose and just before the second dose,4 experts of the Joint Committee on Vaccination and Immunization states that each dose only begins to take effect after several days. Therefore, they interpreted data from the period immediately after the second dose as indicative of efficacy of the first, increasing significantly this 52% protection measured.5 Food and Drug Administration also pointed that efficacy against severe COVID-19 occurring after the first dose was 88.9% (95% CI 20.1, 99.7).6
The basic reproduction number (R0) of SARS-CoV-2 infection have been estimated to range 2 to 6. With an estimated R0 of 3, the herd immunity threshold is about 67%.7 Two ways to reach this desirable percentage group immunity are natural immunization after infection and mass vaccination of the population.
In Spain, the large longitudinal sero-epidemiological population-based study ENE-COVID estimated that the global prevalence (percentage of population with IgG antibodies against SARSCoV-2 since the beginning of the study in April, to November 2020) is 9.9% (IC95%: 9.4–10.4).8 Nevertheless, this information must be taken with caution: not every infected person develops antibodies, and these antibodies can disappear over time. But these do not necessarily mean an absence of immune memory, since cellular immunity seems to play an important role on protection.9
More than 47 million people live currently in Spain. With the aforementioned reported vaccine efficacy of 95%, to achieve 67% of the immune population at least 33 million citizens would have to be vaccinated, which means 66 million doses of vaccine.
Pfizer has committed to deliver to Spain 350,000 doses every week. That means we would need 95 weeks (1.8 years!) to reach the herd immunity in our country, which is definitely unacceptable. This excessively long-time frame could be improved by increasing the number of doses supplied to Spain by the laboratory (assuming that there is enough production capacity) and/or authorizing the administration in Spain of vaccines from other laboratories apart from Pfizer-BioNTech (as Moderna, AstraZeneca, J&J, Sanofi and Curevac), together with accelerating the negotiation process for their acquisition.
In the meantime, one possible strategy might be to administer a single dose of vaccine to as many eligible individuals as possible, delaying the second dose. This will have the greatest impact on reducing mortality, severe disease and hospitalizations, protecting our overloaded National Health System. But the real fact is that the protection from a single dose of Pfizer-BioNTech's vaccine has not been definitively tested because the study was not designed to assess the efficacy of a single-dose regimen. This raises questions about if it is less effective to receive the second dose a few months later than recommended schedule or how long the protection from a single dose will last.
The dilemma of whether to vaccinate fewer people with the best protection possible or provide twice the number of people with a single shot, covering more of the population but with slightly weaker protection should be weighted soon. Now, more than ever, numbers matter.
