Percent efficacy is the most important factor in evaluating new vaccines against covid-19. But how is it measured? What does it depend on?
– How is effectiveness measured?
When a vaccine would be X% effective against COVID-19, it would reduce the risk of contracting the disease by X%, as calculated by the manufacturers based on clinical studies.
Of the tens of thousands of volunteers in the studies, some will receive the vaccine and some will receive a placebo. During the trial, they all lead normal lives and some naturally contract COVID-19. If the vaccine is effective, the number of infected persons will be less among the vaccinated participants.
For example, the Russian vaccine Sputnik V is 91.6% effective against symptomatic manifestations of the virus, according to results published Feb. 2 in the medical journal The Lancet.
The Russian researchers determined this percentage as 16 volunteers of the 14,900 who received the two doses of the vaccine were positively diagnosed, compared with 62 of the 4,900 who received a placebo.
The most effective vaccines to date are those from Pfizer / BioNTech and Moderna (95% and 94.1%, respectively), which use messenger RNA technology.
– What does it depend on?
Efficacy depends on various criteria, such as how the vaccine is administered.
For example, AstraZeneca and its partner, the University of Oxford, stated that the effectiveness of their medications varies based on how the doses are distributed.
“The efficacy reaches 82.4% with an interval of 12 weeks or more between both doses,” compared with 54.9% for less than six weeks, according to a study published Feb. 1.
The European Medicines Agency (EMA) established an efficacy of “approximately 60%” at the time of authorization of this vaccine on January 29, with an interval of between 4 and 12 weeks.
The severity of the disease can also be a factor. Thus, the effectiveness of the Johnson & Johnson vaccine is generally 66%, but specialists emphasize the fact that it increases to 85% when it comes to preventing serious manifestations.
Scientists also wonder whether it would be possible to increase the effectiveness of vaccination by using a different antigen for the second dose than the first.
The University of Oxford has just started a study with 820 volunteers over the age of 50 to test this hypothesis with the Pfizer / BioNTech and AstraZeneca vaccines.
– Efficacy against variants?
Scientists are concerned about the South African variant, as one of the mutations, called E484K, is likely to make vaccines less effective.
South Africa decided on Wednesday to use Johnson & Johnson’s vaccine, not yet approved in Europe and the United States, ahead of AstraZeneca’s, after a study questioned its effectiveness against this variant.
The study, conducted on 2,000 people, concludes that the Anglo-Swedish pharmaceutical vaccine provides only “limited protection against the moderate manifestations of the disease caused by the South African variant in young adults”.
However, many experts warned against drawing definitive conclusions, and the WHO on Wednesday recommended using this vaccine “even if there are variants.”
The study is “concerning” but has “significant limitations,” said WHO Director-General Tedros Adhanom Ghebreyesus, citing the small number of participants and the fact that they were young and healthy and therefore not representative of the general population.
Pfizer / BioNTech, for their part, published a study Tuesday in the journal Nature Medicine noting that their vaccine is effective in vitro against large mutations of the British and South African variants.
Moderna said its vaccine is effective against the British and to a lesser extent the South African, so it will develop a second dose specifically targeting the second.
A study published Wednesday in Nature nevertheless concluded that the Pfizer / BioNTech and Moderna vaccines lose a “small but significant” portion of their efficacy to the variants.
Messenger RNA vaccines may need to be periodically adjusted to avoid loss of efficacy, their authors conclude.
– Effective for who?
It is not known whether these vaccines are equally effective in older people, as the immune response decreases with age.
In particular, there are doubts about the AstraZeneca vaccine, and several European countries only administer it to people under 65 or even 55 years of age, due to a lack of data.
However, the WHO ruled on Wednesday that this antigen also applies to this age group.
On the other hand, it remains to be determined whether these vaccines prevent the transmission of the disease, in addition to protecting the vaccinated person.