Covid vaccines: how protected are you after a dose?

What degree of immunity does this first injection offer?.At the time of writing, over 20 million Italians have received at least the first dose of one of the anti-covid vaccines available ( here the data is continuously updated ). At a time when the increased risk posed by reopening needs to be balanced with the relief of at least partial protection from the disease , what coverage does a single dose of the various types of vaccines offer? Here we resume a summary of the data that emerged both in the final phase of large-scale trials (trials) and in the real world, as reported in an article recently published on Business Insider .

HOW TO INTERPRET THE PERCENTAGE OF EFFECTIVENESS. The vaccine efficacy rate indicates how many fewer people became ill in the vaccinated group than in the control group during the trial. We therefore refer to the relative reduction of the risk of contracting symptomatic covid, in this case, in the vaccinated group compared to the placebo group. To say that, for example, a vaccine is 95% effective, does not mean that 5% of those who received the vaccine did not develop protection, but that in the vaccine group there were 95% fewer patients. compared to the control group.

These estimates developed during clinical trials do not always coincide perfectly with the real-world efficacy, which becomes known only after several months of studies on millions of vaccinated people. The second dose of vaccine is however fundamental : in people who already acquire complete protection after the first dose, the second dose guarantees a better quality and duration of the immune defenses. Among those who have not acquired complete protection after the first dose, some develop it after the second and some do not – there are people who, due to an impairment of the immune system, do not have sufficient defenses in place even when fully vaccinated.

PFIZER-BIONTECH VACCINE: AT LEAST 80% EFFICACY. According to US FDA documents, based on phase 3 trials, Pfizer’s vaccine offers 52.4% efficacy from covid with symptoms between the first and second doses. However, this percentage also includes the 11 days before the usual two weeks necessary to trigger an adequate immune response, so the effectiveness after that threshold could be higher.

According to Stephen Evans, a professor of medical statistics at the London School of Hygiene & Tropical Medicine once on the EMA’s drug safety committee, real-world studies indicate quite clearly that a single dose of Pfizer’s vaccine induces protection. at least 80% and probably better than 90% , although it is not clear what happens after the canonical 21 days for the second dose, because this has not been fully tested. Protection from hospitalization and death should be 100% even after the first dose, although the numbers justifying this claim are very small.

MODERNA VACCINE: AT LEAST 80% EFFICACY. Moderna’s vaccine is expected to offer 69.5% efficacy between the first and second doses, although this includes the 13 days before full protection is achieved and therefore, the final percentage may be higher. It is not clear how much a single dose protects against more severe cases, because few people in the control group have contracted the disease in a severe form and it is not possible to make a meaningful comparison. During the trials, 7% of the vaccinated did not receive the second dose: in these people, protection from symptomatic covid was 50.8% up to 14 days after the first dose and 92.1% after 14 days.

Again, for Evans, real-world data shows at least 80% and probably better than 90% efficacy against symptomatic disease after a single dose, for 28 days (until the ideal booster time). After that threshold there is no data available.

OXFORD-ASTRAZENECA VACCINE: MORE THAN 70% EFFICACY. Although large-scale data for AstraZeneca’s vaccine are more uncertain, due to the different experimental implants followed in phase 3 trials , a single dose would have 76% efficacy , according to a major review published in February in the Lancet . against symptomatic covid for at least 90 days, while the protection against hospitalization and death would be 100% (even here the numbers for a comparison are however small). For Evans, according to available studies, a single dose offers at least 70% protection, while after 90 days there is still not enough data.


ScienceNeutralizing antibodies and vaccine efficacy


JOHNSON & JOHNSON VACCINE: 66% EFFECTIVENESS. In the trials of Johnson & Johnson’s single-dose vaccine, efficacy against moderate to severe disease was monitored, not symptomatic covid as in other vaccine trials. Furthermore, the tests of this vaccine were the first to verify its efficacy against variants as well. The protection is triggered on the 14th day and reaches 66.1% efficacy at 28 days. The percentages vary according to the prevailing variant: when the tests were carried out, the effectiveness was 72% in the USA, 64% in South Africa, 68% in Brazil .

A SINGLE DOSE GREATLY REDUCES THE CHAIN OF INFECTIONS. In addition to the importance of not being directly infected, there is also the possibility, with one’s own vaccine, to protect family members and friends who have not yet been vaccinated. Studies carried out in the UK show that one dose of Pfizer and AstraZeneca vaccines reduces infections by 65%. Furthermore, even those who are infected contract the disease with a reduced viral load and run a half risk of transmitting it in turn ( we have written about it extensively here ). Studies underway in the United Kingdom, where the Indian variant is spreading, however, show that both doses of vaccines are needed to more effectively combat the coronavirus variants.

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