By Alice Han, Edited by Kathleen Navas & Sami Morse
In the race to vaccinate, the Pfizer, BioNTech, and Fosun Pharma candidate, BNT162b2, just took one huge step towards the finish line. According to interim analysis of their phase III clinical trials, researchers have found the vaccine to be more than 90% effective in preventing SARS-CoV-2 infection. This number is substantial; in August, experts speculated that a vaccine that provided immunity to 50% of patients would be a “game changer”, and for comparison, the influenza vaccine hovers around 40% to 60% efficacy. The preliminary results from BNT162b2’s clinical trials are extremely encouraging, and assuming approval and high vaccination rates, this vaccine may lead us out of the pandemic. Furthermore, if successful, it may usher in a new era of mRNA vaccine development, being the first of its kind administered on human populations.
Now, Pfizer, BioNTech, and Fosun Pharma must submit their vaccine for Emergency Use Authorization (EUA) approval with the U.S. Food and Drugs Administration (FDA). In normal circumstances, the approval process of all drugs, biologics, and vaccines are overseen by the Center for Biologics Evaluation and Research (CBER) of the FDA, with an approval process divided into two tiers: Standard Review and Priority Review; Priority Review may take up to six months to be fulfilled and the Standard may take up to ten. However, given the dire situation of the global pandemic, the FDA has authority over EUAs, which gives the commissioner the power to allow the limited distribution of unapproved medical products.
The companies made a statement on their planning EUA submission to the FDA after they achieve the required safety milestone. The submission is expected to occur during the third week of November and be cleared by as early as mid-December, if following the current progress trajectory.
As determined prior to the initiation of phase III clinical trials, they will continue their study until it reaches 164 cases of the coronavirus among its participants, in order to “collect further data and characterize the vaccine candidate’s performance against other study endpoints,” according to a Pfizer statement.
These results should be read with cautious optimism, however. Based on the procedure listed in their report, Pfizer determined 90% effectiveness based on a small sample size; the total pool of 43,000 participants was divided evenly between patients receiving two doses of the vaccine and patients receiving an inactive placebo in a double-blind study. However, the 90% statistic reported does not come from proof that 90% of the vaccine recipients are immune to viral challenge. Directly infecting study participants with SARS-CoV-2 is ethically questionable, so the large number serves two purposes:
1) demonstrating vaccine safety
2) increasing the chances that some of these participants randomly catch coronavirus.
Once enough patients to prove statistically significant vaccine efficacy have been infected with the virus (statistical discussion on page 111 of the same report), the researchers check the ratio between ill vaccine recipients and ill placebo recipients. Using Bayesian statistics based on a beta-binomial model, they can then determine effectiveness.
Thus far, only 94 participants have caught Covid-19, and 8 of these were vaccine recipients, compared to 86 placebo recipients. At first glance, this data looks promising for BNT162b2. At 164 ill participants, we have 90% power to conclude true vaccine efficacy; in less statistical terms, this means that if we use this data to conclude the vaccine is effective, we are probably correct. While we only need a very small percentage of the total Phase III population to get sick before we can prove efficacy with statistical significance, we are not quite at that point yet. Personally, I would recommend staying excited about this vaccine, but not treating the interim reports as definite.
If this vaccine is approved and brought to the market, we still must face the vaccine’s unique challenges of distribution, encourage a large proportion of the population to take the vaccine, and listen to public health experts. If we can successfully clear these hurdles, we may finally be out of the woods.
We also suggest continuing to keep an eye on the Moderna and Novavax vaccines, which don’t need to be refrigerated like Pfizer’s, because if these prove similar efficacy without the distribution challenges, they might end up being more viable. (Students vs Pandemics at Berkeley will be publishing updates on both of these this week, so keep checking our social media posts about this!)
Results from ongoing research and the current understanding of COVID-19 are constantly evolving. This post contains information that was last updated on November 15, 2020.
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