March 7, 2021

The positive surprises of the AstraZeneca vaccine, pending conclusive results on its efficacy in people over 55 years of age

The Public Health Commission of the Interterritorial Council of the National Health System has chosen this Friday for not administering the Oxford-AstraZeneca vaccine to people over 55 years of age and it will reserve the first doses of this drug, which will arrive in Spain during the weekend, for active healthcare and social health professionals – who are not first-line. What do we know about her?

AstraZeneca’s vaccine has a “big impact” on reducing transmission

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This week, an update with new vaccine data has been posted as preprint —Not reviewed yet by other specialists— in the magazine The Lancet. The results confirm the global efficacy of the vaccine, similar to December results, but they also include surprises.

The first dose does not lose activity against symptoms and severe disease until at least 90 days have passed, which would allow the immunized population to be extended before needing a second dose. Against what might be expected, the effectiveness seems even higher if you wait those three months. In addition, the number of people with PCR positive was reduced by about half, suggesting that it could help slow transmission. This Friday came another good news, preliminary tests show that this vaccine is effective against the so-called British strain of the virus.

Against serious illness

The first publication of the essay included 131 positive cases, now 201 more have been added. At approved doses, the vaccine is 63% effective in preventing symptoms of the disease, similar to 62% in December. But the most encouraging piece of information is that, so far, it completely prevents severe disease. After three weeks of the first dose, 15 volunteers had to be hospitalized, and all 15 were in the control group.

“That is very good news,” says Marcos López Hoyos, head of the Immunology Service of the Hospital Marqués de Valdecilla (Santander) and president of the Spanish Society of Immunology, “that is the main objective of vaccines, after all “.

The surprises came when the data was broken down. The United Kingdom took the sudden decision to increase the separation between the doses of the vaccines up to three months, which prompted the regulators to ask for a specific analysis of what it could mean in terms of risk and efficacy. The development of the trial, somewhat stumbled, made it easy.

A win-win? The surprise of the intervals

Production delays, the fact that the trial was originally designed for a single dose – some patients later refused the second – and the UK’s own decision meant that more people than expected to stick with a single dose longer than initially planned. The analysis of these volunteers found that the efficacy, after the first three weeks, was 76% up to three months.

“It’s something strange,” Hoyos acknowledges. “It does not make sense that the efficacy seems greater with one dose than with two. It is probably not the case, and the result has to do with the type of analysis – ex post trial design, which tends to reduce reliability – and the groups that They are compared. On the other hand, after 90 days there does seem to be a drop in efficacy, although the numbers are small. We need more data in this regard. In any case, it seems that immunity does not decline until at least three months have passed , which is positive. And, of course, that in no case means that you have to give up the second dose. ”

The next surprise came when analyzing the efficacy according to the waiting time until the second dose. If the interval was less than 6 weeks, it was 55%. On the other hand, if it was done at 12 weeks or more, it rose to 82%. In other words, waiting 90 days to obtain the booster would not only increase the number of immunized people, but the response obtained seems even greater. The strategy has overtones of win-win.

Now, how can these results be explained? Why does waiting longer increase efficiency? “Honestly, I don’t have an explanation,” Hoyos confesses. “The researchers cite that it is something that has already been seen with other vaccines such as influenza or malaria, but in principle if 3-4 weeks have already passed it should not be very different. In any case, it tends to increase with each interval studied, so it seems consistent. ”

With all these data, the Public Health Commission of the Ministry of Health has decided establish an interval of 10 to 12 weeks between the two doses.

About transmission

One of the conclusions that have been published from preprint is that the vaccine reduces the transmission of the virus by 67%. However, this is data that we still do not know and that cannot be directly extrapolated from the trial.

In fact, that number indicates that, in the group of volunteers who received the first dose of the vaccine, there were 67% fewer cases with positive PCR than in the group of volunteers who received placebo. In the case of those who had received two doses, the percentage was 49% (which seems more reliable, due to the greater number of cases and because it responded to the original design).

To obtain these data, they took advantage of the fact that, in the cohort from the United Kingdom (there were also from Brazil and South Africa), the volunteers were asked to take a weekly test themselves. Thus they were able to detect not only obvious infections, but also those that could pass asymptomatically. The vaccine did not alter the number of infections that passed without symptoms, but it did decrease the number of total infections.

In good logic, if you reduce the positive cases, you should also reduce the possible infections. However, although it is very likely that it will, we cannot be certain, least of all the exact number. “We have not specifically measured transmission because that is a different type of study,” he clarified. Andrew Pollard, director of the Oxford Vaccine Group during a press conference on Wednesday. “What we have is a study that indicates the number of people who are no longer infected and if you are not infected, you are not supposed to transmit the virus.”

Some real life variables might work in your favor. For example, if those vaccinated who pass the disease do so with fewer symptoms, they could be less contagious. But we must also consider that the tests in asymptomatic patients were carried out by the volunteers themselves and only half presented the results, which could alter the numbers of real infected.

The opinion in the magazine Stat from Kathleen Neuzil, director of the Center for Vaccine Development at the University of Maryland, is that “there are too many variables to draw too many conclusions from this result. However, the trend is consistent and going in the right direction.”

This is how López Hoyos also considers it. In addition, taking into account not only the reserves, but the fact that the vaccine seems to reduce infections but far from eliminating them, he emphasizes that “at least until reaching group immunity, vaccinated people should follow the same precautionary measures what if they weren’t. ”

In general, “it would be good to expand the data on some aspects, and of course it is convenient to wait for the review of the article by other specialists”, summarizes López Hoyos. “For example, we still do not have much information on its results in people over 65 years of age, so it might be advisable to initially administer it to younger people. But overall, it seems safe and effective, especially on severe disease, which after all, it is the most important thing. ”


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