By Alexandra Tauziac – [email protected]
To avoid catching Covid, a single dose of vaccine is not enough. But what are the effects of a single dose on severe forms and on the vaccination schedule?
For several weeks the government and health authorities have been insisting on the importance of being fully vaccinated to block the Delta variant of the coronavirus, which is more transmissible and more aggressive. Because the studies are clear: to be effectively protected, you need two doses.
For now, nearly a third of French people are fully vaccinated (either with two doses, or with only one for those who have already had the Covid or have received the Janssen vaccine). And almost half (49.5%) received a first dose. But in their case, this is not enough, especially since it takes 15 days to develop immunity.
What protection after a single dose?
According to a study published in The Lancet, after a single dose of Pfizer / BioNTech, mainly administered in France, 79% of people develop antibodies against the original strain of the virus. But this falls to 50% for the Alpha (British) variant currently in the majority in France, 32% for the Delta (Indian) variant, which is gaining ground should be in the majority everywhere in Europe by the end of August, and 25% for the variant. Beta (South African).
After a single dose, the AstraZeneca vaccine is also only 49% effective against the Alpha variant and 31% against the Delta variant, according to a study by Public Health England (PHE). As for Moderna, also used in France and the United Kingdom, it was still too little administered to be included in the research.
The fact remains that the low efficacy of a single dose, in particular against emerging variants, explains why it is increasingly common to see first-time vaccines contract Covid.
Fewer serious forms despite everything?
On the other hand, even with a single dose, vaccines help limit severe forms and therefore hospitalizations. Always according to a study by Public Health England, Pfizer / BioNTech would be 83% effective in preventing severe forms of the Alpha variant after a first injection (against 95% with two doses) and at 94% for the Delta variant (96% with two doses).
As for AstraZeneca, it appears to be 71% effective against severe forms of the Delta variant after a single dose (92% with two) and 76% against hospitalizations linked to the Delta variant (86% with two doses).
Should I be revaccinated anyway?
If people who contracted Covid before the vaccination campaign only need a single dose to develop greater immunity than the two doses injected into a person without a history, what about those who catch the Covid between the two doses? It all depends on how long after the first dose the person gets the virus.
In the event of an infection occurring less than 15 days after the first dose, a second injection is recommended, starting two months after the infection. But if the infection occurs more than 15 days after the first dose, the Vaccine Strategy Orientation Council (COSV) considers it equivalent to a second dose of vaccine, so immunity is sufficient.
–
