Pharmaceutical Pfizer said Monday that its vaccine against COVID-19 demonstrated an “efficacy rate of over 90%.” This soon translated into a 90% “effectiveness” in preventing the new coronavirus. But what is the difference?
November 12, 2020 Share on FacebookShare Share on TwitterTweet Share on WhatsAppShare
The term efficacy refers to the protection of the vaccine measured in RCTs normally under optimal conditions where storage and distribution of vaccines are monitored and participants are generally healthy (Bloomberg)
There was, rightfully, a lot of excitement when Pfizer and BioNTech announced the interim results of their COVID-19 vaccine trial. The vaccine, called BNT162b2, was reported to have a “greater than 90% efficacy rate.” This soon translated into a 90% “effectiveness” in preventing the new coronavirus. But what is the difference?
“ Efficacy is the percentage reduction in the incidence of a disease in subjects who were vaccinated with respect to the general who was not vaccinated. The effectiveness is actually the ability of the vaccine to protect against the disease when the vaccine is already on the market ”, explained in dialogue with Infobae Ricardo Teijeiro, infectious disease doctor member of the Argentine Society of Infectology (SADI).
The Pfizer / BioNTech vaccine reports 90% efficacy, which means that its vaccine prevented COVID-19 symptoms in 90% of volunteers who received the vaccine compared to placebo. This is very high and will likely change at the end of the study. The press release reported the results of 94 participants – they need 164 to complete the test, which shouldn't take long. Safe vaccines with an efficacy greater than 50% are expected to be approved to combat COVID-19.
“Talking about the effectiveness of this vaccine is -for now- incorrect. We must wait for it to be used in a massive way to be able to do so. However, they are very hopeful vaccines that are showing much greater efficacy than expected, ”said Eduardo López, a prestigious infectious disease specialist and one of the President's main advisers in the fight against coronavirus in our country.
The effectiveness of a vaccine refers to the protection that the vaccine provides as measured in observational studies that include people with underlying medical conditions who have been receiving vaccines from different healthcare providers under real-world conditions (Bloomberg)
Clinical trials are precise and thorough, and they aim to answer whether a vaccine is safe and whether it works. To achieve this, participants who are recruited to receive the vaccine (or a placebo) are likely to be generally healthy. In early clinical trials, participants may not be the vulnerable group of people we intend to protect with this product, for example children or older people with other conditions.
To determine the efficacy of the vaccine, we must compare it with a “control” treatment, which is usually an irrelevant or known vaccine or a similar preparation that should not work for the tested virus. Trials are often “double blind,” so participants do not know which vaccine they received and researchers do not know which vaccine they administered until the end of the study.
So what do we understand by vaccine efficacy? Vaccine monitoring does not stop once they are approved for use. When the vaccine is deployed, data will continue to be collected to study how well it works over the years for all vaccinated people.
Important parameters include vaccine performance for different groups (age, ethnicity, other conditions), duration of protection (duration of immunity and effectiveness against evolving virus strains), balance of benefit versus harm . Cost-effectiveness is also an ongoing consideration, based on comparisons with other treatment options and vaccines.
We do not know what the overall effectiveness of the vaccine will be in preventing COVID-19 symptoms, serious illness or death, and it may take several years before studies report the effectiveness of BNT162b2 for different groups (REUTERS)
The effectiveness of a vaccine is considered to be the results or health benefits provided by a vaccination program in the target population, when the vaccines are administered in the real or usual conditions of daily healthcare practice or program development. However, good efficacy does not always imply good effectiveness. For Pablo Bonvehí , infectious disease physician (MN: 62.648), head of the Infectology and Infection Control Section of CEMIC, “the effectiveness of a vaccine is determined once the vaccine is approved and begins to be administered.”
In this context, in dialogue with Infobae , the infectologist Lautaro de Vedia, former president of SADI, indicated: “Although they are terms that may sound similar, they contemplate very different definitions. Efficacy is measured under ideal experimental conditions: the result of a study that concludes a certain percentage of efficacy. The effectiveness is the same but outside the research study, without controlled variables and in real life ”.
The effectiveness depends on factors such as the acceptance and accessibility of the population to the vaccine, the correct schedule of administration (dose, route, place, technique), proper storage and handling, etc. We don't know what the overall effectiveness of the vaccine will be in preventing COVID-19 symptoms, serious illness, or death, and it may take several years for studies to report the effectiveness of BNT162b2 for different groups. However, it is unlikely to be 90%.
But very few vaccines, other than measles and chickenpox, are 90% effective. The flu vaccine is 40-60% effective, but it still saves millions of lives. And that's something to celebrate.
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