Coronavirus: what does the percentage of effectiveness of vaccines mean and how is it measured?

Coronavirus: qué significa el porcentaje de efectividad de las vacunas y cómo se mide

Covid-19 vaccine effectiveness figures are piling up in the headlines, raising hopes for a world eager to overcome the nightmare of the pandemic.

The last known figure was the 92% efficacy that an article published in the scientific journal The Lancet attributes to the Russian vaccine Sputnik-V, which adds to the 95% reported by the one developed by Pfizer-BioNtech, 94.1% of that of Moderna and 70% of that of the University of Oxford and AstraZeneca.

These are all encouraging data that suggest that mass vaccination programs should serve to bring the SARS-Cov-2 coronavirus under control, which has already caused more than two million deaths in the world, and an unprecedented health and social crisis in recent history.

Unlike those developed by Pfizer and Moderna, which are based on a novel technology that allows the body to be exposed to a fragment of the coronavirus RNA, Sputnik-V uses a common cold virus that is modified to act as a vector and provoke a controlled response of the immune system that will be repeated if it detects the presence in the body of SARS-Cov-2

According to figures from “Our world in data”, more than 103 million people have already been vaccinated around the world, the vast majority in developed countries.

According to experts, many more will be needed to control a pandemic that has accumulated almost 104 million confirmed cases and more than 2.24 million deaths.

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Some of the approved vaccines require two doses.

Although there is still no conclusive data on how long people who receive the different vaccines are immunized against contagion, scientists insist that vaccinating as many as possible as soon as possible is the priority at this time.

The expert infectologist María Elena Bottazzi, from the Baylor College of Medicine in Houston, in the USA, told BBC Mundo that “you don’t have to compare vaccines; You have to get the vaccine that is available in the place where you live. The important thing is to get vaccinated because that will greatly reduce the risk of having a serious illness or dying, and it will help us to start controlling this virus.

Although the information available is still scarce, due to the early stage in which vaccination is found worldwide and there is also no conclusive evidence on the extent to which vaccines prevent the spread of the virus in asymptomatic patients, the first indications point to that the high effectiveness of vaccines contributes significantly to the reduction of infections.

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This has been observed among those over 60 years vaccinated in Israel, where of the more than 750,000 people of this age group who received the vaccine, only 0.07% later tested positive for coronavirus.

According to Dr. Bottazzi, “the high effectiveness that vaccines are showing is the best news we have so far.”

Comparative chart of vaccines.

Each vaccine has different characteristics.

The truth is that when researchers around the world were working in laboratories in search of a vaccine against covid, among the scientific community it was considered that it would be a success to achieve one that was 50% effective and few predicted that it would be obtained one in just a few months, since that is a process that usually takes years.

Botazzi recalls that “most flu vaccines are around 40% effective and still save millions of lives each year”.

But how is the effectiveness of a vaccine really measured?

Efficacy and effectiveness

Although they may seem synonymous, for scientists the effectiveness and efficacy of a vaccine are not the same thing.


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What do the effectiveness rates mean?

As explained by the Centers for Disease Control of the United States (CDC, for its acronym in English), “the efficacy and effectiveness of a vaccine measure the proportional reduction of cases among vaccinated people.” But the term efficacy is used when it refers to “a study that is carried out under ideal conditions, for example during a clinical trial.” The term effectiveness is the one used in “a study that is carried out under typical environmental conditions, that is, less controlled”.

This explains why many experts predict that the unexpectedly high percentages of efficacy that approved vaccines have shown in laboratory trials will decrease when they are applied in the real environmental conditions of the population, when they stop talking about efficacy to begin to do it of effectiveness .

Even so, they will continue to be, together with social distancing, the best weapon that humanity has in its war against the virus.

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Doctors David Spiegelhalter, from the University of Cambridge, in the United Kingdom, and Anthony Masters, from the British Royal Statistical Society, proposed in an article published in The Guardian a way to visualize what we are talking about when it comes to the efficacy of vaccines .

“Imagine 100 people with covid. An efficacy of 90% means that if they had received the vaccine, only 10 would have fallen ill. The efficacy of the vaccine is the relative reduction of risk: whatever your risk, it is reduced by 90% if you are vaccinated ”.

Researchers arrive at these numbers in trials by comparing numbers between groups of vaccinated and unvaccinated people. That is why in the trials there are volunteers who receive the vaccine in tests and others only a placebo substance, and no one knows what it was that was actually administered.

The vaccines that so far have published data on their effectiveness have been tested in tens of thousands of people, and have not presented major safety problems or unexpected adverse reactions have been reported.

In the trial of the vaccine developed by Pfizer-BioNtech, 8 cases were recorded among the 22,000 people who received the vaccine. Among the other 22,000 volunteers who received the placebo, the number of infected climbed to 162. That means that the risk of falling ill among the vaccinated population was 0.04%.

In any case, as the conditions in the field will never be the same as those recreated by the researchers in the laboratory, the effectiveness of the vaccines should continue to be evaluated, not their efficacy. Dr. Bottazzi points out that “we will have to continue monitoring, also to develop new schemes in the event that effectiveness is reduced in the face of new virus mutations.”

And remember: “Scientists are already working on that.”

Child coughing on a bus.

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The distribution of vaccines to the least developed countries is one of the great challenges to end the pandemic.

What will happen now with Sputnik-V

Approved vaccines vary by country. So far, those by Pfizer-BioNtech, Moderna, and the one produced by the University of Oxford and AstraZeneca have received approval in the United States, the United Kingdom and the European Union.

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The Chinese of CanSino, Sinopharm and Sinovac have been approved in China and, in some cases, also in other countries such as Brazil or the United Arab Emirates. In India the emergency use of that of the local manufacturer Bharat Biotech has been authorized.

Sputnik-V aroused misgivings among some in the Western scientific community because they appreciated a lack of transparency in the practices of the Gamaleya Center, the Russian laboratory that developed it. But now now that a prestigious scientific journal has endorsed its results, it is possible that its use is also approved by some countries that until now did not consider it as an option

Map of the world with an injector.

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Experts estimate that 60-70% of the world’s population needs to be immune to the virus to prevent it from spreading further.

In Latin America, several governments have signed agreements to use it, such as those of Venezuela, Argentina and Mexico.

Dr. Bottazzi points out an area in which the Russian vaccine does not seem to measure up to the Western ones based on modified RNA. “It offers high protection, but compared to others the number of neutralizing antibodies it induces is quite low.” This, the expert points out, could harm the duration of the immunity it provides and its potential effectiveness against new mutations of the virus.

However, the expert recalls that no conclusive information yet on how long immunity lasts of other vaccines and the addition of Sputnik-V to the medical arsenal against the disease should be seen as good news.

As the Mexican Secretary of Health, Hugo López-Gatell, said at the press conference in which he announced his approval: “This is encouraging; we have a new vaccine in the repertoire ”.

Taking into account that the main challenge now is to produce vaccines in sufficient quantity and distribute them also in developing countries, where many times there are not adequate conditions for their conservation and handling, the more that are added to the list, the greater will be the hope of ending the pandemic.

*With information from Carlos Serrano.

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