Efficacy expressed as a percentage is the main factor for the evaluation of new covid-19 vaccines. But how is it measured? Depending on what?
– How is effectiveness measured?
When a vaccine is said to be X% effective against COVID-19, it means that it reduces the risk of contracting the disease by X%, calculated by its manufacturers based on clinical trials.
Of the tens of thousands of volunteers in the tests, one receives the vaccine and another receives a placebo. During the process, they all lead a normal life and some of them naturally contract COVID-19. If the vaccine is effective, the number of infected people will be lower among the vaccinated participants.
For example, the Russian Sputnik V vaccine is 91.6% effective against the symptomatic manifestations of the virus, according to the results published on February 2 in the medical journal The Lancet.
Russian researchers determined this percentage because 16 of the 14,900 volunteers who received the two doses of vaccine were diagnosed positively, compared to 62 of the 4,900 who received a placebo.
The most effective vaccines so far are those from Pfizer / BioNTech and Moderna (95% and 94.1% respectively), which use RNA messenger technology.
– Depending on what?
The effectiveness depends on several criteria, such as how the vaccine is administered.
For example, AstraZeneca and its partner, Oxford University, said that their effectiveness varies depending on the dose spacing.
“Efficacy reaches 82.4% with an interval of 12 weeks or more between the two doses,” compared with 54.9% for less than six weeks, according to a study published on February 1.
The European Medicines Agency (EMA) established the efficacy of this vaccine on 29 January with an efficacy of “around 60%” between 4 and 12 weeks.
The severity of the disease can also be a factor. Thus, the effectiveness of Johnson & Johnson vaccine is generally 66%, but experts point out that it increases to 85% when it comes to preventing serious manifestations.
Scientists also question whether it would be possible to increase the effectiveness of vaccination by using a different antigen for the second dose than the first.
Oxford University has just launched a study of 820 volunteers over the age of 50 to test this hypothesis with the Pfizer / BioNTech and AstraZeneca vaccines.
– Effectiveness against variants?
Scientists are worried about the South African version, because one of its mutations, called E484K, will make vaccines less effective.
South Africa decided on Wednesday to use the Johnson & Johnson vaccine, which has not yet been licensed in Europe and the United States, before AstraZeneca, after a study questioned its effectiveness against this variant.
Carried out among 2,000 people, the study concludes that the Anglo-Swedish pharmaceutical vaccine offers only “limited protection against moderate manifestations of the disease caused by the South African variant among young adults.”
However, many experts have warned against drawing definitive conclusions and the WHO on Wednesday recommended the use of this vaccine “even if there are variants”.
The study is “worrying” 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 in good health and therefore not representative of the population. general.
For their part, Pfizer / BioNTech published a study in the journal Nature Medicine on Tuesday that states that their vaccine is effective in vitro against major mutations in British and South African variants.
Moderna has indicated that its vaccine is effective against the British and, to a lesser extent, the South African, so it will develop a second dose specifically aimed at the second.
However, a study published in Nature on Wednesday concluded that Pfizer / BioNTech and Moderna vaccines are losing a “small but significant” share of their effectiveness compared to variants.
“Messenger RNA vaccines may require periodic modifications to avoid loss of efficacy,” their authors conclude.
– Effective for whom?
It is not known whether these vaccines are equally effective in the elderly, as the immune response decreases with age.
In particular, there are doubts about the AstraZeneca vaccine and several European countries administer it only to people under 65 or even 55, due to lack of data.
However, the WHO decided on Wednesday that this antigen is also valid for 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.