The Pfizer SARS-CoV-2 Vaccine

Diagram of immune responses to the virus

Pfizer released some data in a press release indicating that the company's SARS-CoV-2 vaccine is about 90% effective in reducing infections in the tested patients so far. Additional data will be generated in the ongoing clinical trial, so the final efficacy data could be different than what was released. The data has not been subject to peer review, so this is only preliminary.

The Pfizer vaccine is injected as shown in part b of the figure above. The protection from this vaccine is believed to come from the immune response in the lungs, but not in the nose and upper airway. 

A paper summarizing SARS-Cov-2 vaccines under development make the following observations:

1. It is likely that two doses of a vaccine will be required, with booster doses potentially necessary at later time points; in this case, at least 16 billion doses will be needed to meet the global demand. 
2. For SARS-CoV-2 vaccine candidates, there have so far been no signals of enhanced disease in animal models or in humans; however, such a safety signal would certainly derail the development of a vaccine candidate and would negatively affect vaccine development in general.
3. Participants in the 100 μg group did not receive a booster dose due to tolerability profiles of the 100 μg dose post-prime and the 30 μg dose post-boost.
4. At 14 days post-boost, titres reached 1:180 and 1:437, respectively. Convalescent serum was also tested and reached titres of 1:94. However, it is unknown how representative these sera were.
5. Systemic adverse events after the prime dose seemed to be dose-dependent and included fever—especially in the 100 μg group, for which it was seen in 50% of individuals—fatigue, headache and chills.

What is not yet known is how long immunity will last and how well immunity will protect people in different age groups. Data from one group of older individuals (65–85 years) indicated that the immune response and antibody titres were lower compared to that in younger individuals. The vaccine uses a genetically modified RNA variant of the spike protein that covers the surface of the virus. That protein is what the virus uses to enter human cells. Because RNA in the vaccine is used to generate the immune response in vaccinated people, it has to be stored on dry ice (-78.5ᵒ C, or -109.3ᵒ F). That will create significant logistics complications in manufacturing, distributing and storing the vaccine. RNA is a very unstable molecule due to the ubiquitous presence of enzymes that rapidly break it down. That is why the vaccine needs to be solidly frozen on dry ice. 

The side effects seem to be generally more pronounced for the vaccine compared to most flu vaccines. That makes the vaccine generally less pleasant for most people compared to flu vaccines.

The vaccine works as shown below. The vaccine is injected into muscle and some of the RNV enters into cells where the RNA is translated into the spike protein. The body them mounts an immune response against the spike protein.

 

For context, traditional vaccine development generally takes about 10-15 years. Given the urgency of the threat, development time for SARS-CoV-2 is shooting for a about 10-18 months, assuming something does not go seriously wrong during the development and regulatory review period. Given that accelerated development schedule, the risk of failure to detect a serious side-effect increases, especially a side-effect that takes years to manifest itself.