Why are some places giving one dose instead of two to those who have passed covid-19?

You are asking us why some countries are administering two injections to patients who have already passed covid-19, while in Spain this group is being vaccinated only once. Having a look at the scientific literature on the subject, the monodose strategy in those who have already passed the SARS-CoV-2 infection not only has advantages on a personal level, but is also beneficial for the rest of the population. We explain why.

Although to date more countries such as Spain apply a single dose to those who have already passed SARS-CoV-2 infection and are younger than 65, the political debate is still on the table. The scientific literature is increasingly clear: people who have been infected with SARS-CoV-2 have the capacity to generate antibodies in the long term. And when the body already has its own antibodies against SARS-CoV-2, a single dose confers enough protection, as explained, for example, eight researchers from the University of Maryland in a letter published last 25 May in the Lancet-owned Journal EBioMedicine. 

In the letter, they argue that “the emerging real-world evidence suggests that antibody responses to the first dose of vaccine in individuals with prior SARS-CoV-2 infection is equal to or higher than antibody titres found in inexperienced individuals after the second dose". 

Consensus at a scientific level

Beyond the commentary, there is consensus at the scientific level: “An increasing number of scientific studies indicate that a single vaccine dose in post-infected individuals is enough to generate antibody levels equal or higher than those seen in uninfected individuals who received two doses”, Adelaida Sarukhan, immunologist and scientific writer at ISGlobal, tells Verificat. "It has also been shown that the second dose in people who have already been infected does not provide much benefit (in terms of antibody levels)", she adds. 

It is likely, she adds, that people who passed the infection and received a single dose of vaccine "are better protected, not only because of the level of antibodies, but also because of the diversity". When someone is infected naturally, with the full virus, they generate antibodies and T-cells against different viral proteins, whereas vaccines based on the spicule protein only generate immunity specifically against the Spike protein.

As an example of this, a recent study has shown that antibodies observed in post-infected individuals who received a single dose of vaccine are better able to neutralise new viral variants than antibodies in uninfected individuals who receive two doses of vaccine.

And this, if we are talking about antibodies. If we also focus on the duration of natural versus vaccine-generated immunity, "it is thought that natural immunity will be quite long-lasting," the immunologist notes, although she insists that, first, "it is still too early to know which will be longer" and, second, that this does not mean that it is better to be infected than vaccinated: "The risks associated with infection (death, hospitalisation, persistent covid) are too great, for people of any age," she concludes. "Moreover, people who have already been infected need a dose of vaccine anyway to be well protected against the new variants”.

One dose, better also at the population level

Beyond the personal benefit, the scientists insist that at the population level, especially in the context of limited resources, this strategy is also beneficial: "Changing the current vaccine recommendation to provide only one dose of vaccine to COVID-19 survivors would free up many urgently needed doses of vaccine”.

That would, for example, have a positive impact in regions where the second dose is being delayed due to lack of units: "With the additional vaccines available, there would be no need," the experts in the letter conclude.