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COVID-19 vaccine FAQ: six things to look for in clinical trial results

Hesitation about taking a COVID-19 vaccine decreases as people learn more about vaccine safety testing, so these conversations are important to make informed choices about public health.

As the second wave of COVID-19 escalates in Canada and around the world, pharmaceutical companies have been announcing vaccine trial results that promise to help ease restrictions and return us to pre-pandemic routines next year.

However, there is also a lot of vaccine skepticism, as well as questions about how much confidence should be placed in these announcements. Hesitation about taking a COVID-19 vaccine decreases as people learn more about vaccine safety testing, so these conversations are important to make informed choices about public health. Here are some questions to keep in mind about these vaccine study announcements.

The 11 different Phase 3 clinical trials for COVID-19 vaccine candidates each involve thousands of people getting either a vaccine or a placebo, but not knowing which they received. When some of the participants get COVID-19, researchers compare the number of sick people who had received the vaccine to the number who received the placebo.

The first results announced came from Pfizer/BioNtech and Moderna, companies that produced vaccines based on a new technology that uses a piece of genetic instructions in a fatty nanoparticle to infiltrate living cells. Cells use a snippet of the SARS-CoV-2 spike protein as a blueprint to make virus fragments the immune system learns to destroy. If approved, these will be the first licensed mRNA vaccines.

An effective vaccine needs to reduce the risk of infection in populations who are most vulnerable. COVID-19 has shown higher rates of complications and death in racialized groups, particularly Indigenous, Black, and Latinx populations; people older than 65; and people with medical conditions such as diabetes, kidney disease, and obesity. Lack of representation in clinical trials has been an issue for years, particularly for historically marginalized and exploited groups.

In the Pfizer/BioNTech vaccine study of 43,000 participants in six countries, 42% of participants are Asian, Black, Hispanic/Latinx, or Native American. Of the 30,000 participants in the Moderna trial, 37% are from racialized and ethnic minority groups, and the study included significant representation of people over 65 and people with higher risk medical conditions.

The best vaccines protect against infection in 95% of people, such as vaccines for measles. Influenza vaccine reduces the risk of infection by about 50%, which is the threshold the United States Food and Drug Administration (FDA) set to consider licensing a vaccine.

Pfizer/BioNTech announced a 90% reduction after the first 94 cases. The trial finished after 170 cases with a 95% reduction for people who received the vaccine, including 94% in people over the age of 65. The Moderna vaccine showed 94.5% reduction. Of their 95 symptomatic cases, only five occurred in participants who had been vaccinated.

Like the Pfizer/BioNTech and Moderna vaccines, the Oxford/AstraZeneca vaccine uses an initial dose and a booster. There were 23,000 participants expected to get either a non-COVID control vaccine or a COVID vaccine and booster. Due to a miscalculation, one-quarter of participants who received the COVID vaccine got a half dose followed by a full-dose booster. Participants who got two full doses showed 62% reduction in COVID cases, while participants who received a half dose before the booster showed a 90% reduction.

Because this accidental group is smaller and was a bit younger, this number may not be an accurate reflection of how effective the vaccine is. Of the 131 cases observed in the Oxford/AstraZeneca trial, there was a 70% reduction in people who received either quantity of the COVID vaccine.

The Pfizer/BioNTech and Moderna studies relied on participants having symptoms and getting tested for the virus. The Oxford/AstraZeneca trial had some participants regularly screened for asymptomatic infection. The different study design means it is not entirely fair to compare the percentages between trials. More information is needed before we can say anything about how these vaccines affect asymptomatic infections or transmission.

It is difficult for a COVID-19 clinical trial to demonstrate a reduction in severe illness because severe cases are less common and participants in clinical trials are generally healthy. 

Pfizer/BioNTech have not yet reported any information about the severity of cases. There were 11 serious infections during the Moderna trial which occurred in people who had not received the vaccine. In the Oxford/AstraZeneca vaccine, none of the 131 reported cases was severe. Even once these trials are complete, the numbers remain too small to absolutely say if any of the vaccines prevent severe complications.

Any vaccine that caused severe side-effects in more than 10% of the hundreds of Phase 2 participants would not be allowed to progress to Phase 3. This means that the most common severe side effects, such as allergic reactions that require hospitalization, are unlikely.

Most vaccines, including many COVID-19 vaccine candidates, include additives called adjuvants that produce a stronger immune response linked to more effective, longer-lasting protection. However, their stronger immune response also means vaccines containing adjuvants may be more likely to induce reactions like swelling at the injection site and fever.

If several vaccines are safe and effective, we can consider other factors affecting distribution and vaccination access. The clinical trials reporting results use two doses per person, requiring more vaccine production and follow-up appointments, creating further barriers for the economically disadvantaged.

Some vaccines have different requirements for shipping and storage. The Pfizer/BioNTech vaccine must be kept at -70 degrees Celsius, which is colder storage than most clinics and pharmacies have. It lasts up to five days in a refrigerator. The Moderna vaccine is stable for six months at -20 degrees Celsius, a month in a refrigerator, and up to 12 hours at room temperature. The Oxford/AstraZeneca vaccine, a modified common cold adenovirus, is stable in a refrigerator for at least six months.

A six-month study cannot tell us how long a vaccine will provide protection from the virus. This would be a question with any new vaccine, but mRNA vaccines are a new technology, creating more uncertainty. The Pfizer/BioNTech trial was the largest, with 43,000 participants, but we cannot know about side-effects occurring as infrequently as one in 100,000 people. Based on the severity of side-effects, that could change whether the vaccine benefits are worth the risks.

These trials also cannot tell us how willing people will be to accept a vaccine once it is approved. A vaccine’s effectiveness in improving public health depends on people taking it, which they are more likely to do when they understand the vaccine trial process. — The Conversation

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