There have been predictions by the Trump administration and by some news media as well that it may be possible to have an effective preventive vaccine for the SARS-COV-2 virus by this fall. Every even slightly positive report of phase 1 and phase 2 trials have been breathlessly reported in the media. The purpose of this post is to give some reasonable estimates of when and if there could be an effective and safe vaccine for prevention of SARS-COV-2.

How vaccines are tested and developed

Exploratory stage

In this stage academic and federally funded scientists identify antigens (molecules that stimulate the immune system) that might help or prevent a disease such as COVID-19. This stage normally takes two to four years, but because of the tremendous financial resources and number of scientists involved in the exploratory stage, there are many potential vaccine antigen candidates already identified. In fact there are currently 165 vaccine candidates who have completed the exploratory phase.

Animal Trials

Vaccine candidates are often tested in animal models. If the vaccine protects animals from infection then it is more likely to lead to human trials. Vaccine protection in animals though, does not mean the vaccine will work or will be safe in humans. The two coronavirus vaccines that are farthest along in development, the one by Moderna and the one by Oxford have shown protection from pneumonia in mice and monkeys, but the noses of the animals showed just as much virus as monkeys that were not vaccinated.

Phase 1 Trials

In phase 1 trials, a small group of people are given a vaccine to see if the vaccine induces antibodies and if there are any adverse reactions. These involve only 15-20 people. The production of antibodies does not prove the vaccine will be effecting in preventing or reducing severity of infection. Twenty eight coronavirus vaccines have completed phase 1 trials.

Phase 2 Trials

In phase 2 trials the vaccine is given to hundreds of healthy people in different age and risk groups to see if the vaccine acts differently in different groups of people. Phase 2 trials provide more information about antibody production, but do not provide evidence about the effectiveness of the vaccine in the real world. Fifteen vaccines have completed or are currently in phase 2 trials.

Phase 3 Trials

In phase 3 trials, the vaccine is given to thousands of people. Half the people get a placebo injection while the other half get the vaccine injection. Both groups are followed over time to see how many in each group get infection and how severe the infection is. Each group is also monitored for side effects. A vaccine that is safe and effective will show decreased infection in the vaccine group compared to the control group and side effects in the vaccine group should be minimal and not serious. To count as effective, a vaccine would need to be at least as effective as the flu vaccine, which usually has about 50% protection. Nine vaccine candidates are in phase 3 trials. There are no shortcuts for phase 3 trials. Depending on the prevalence of the virus in the population being tested, phase 3 trials are likely to last from 1 to 4 years.

Chances of success

The success rate for vaccines that make it to phase 3 trials is low, around 30%. The good news is that there are a very large number of vaccines in various stages of testing. It is likely that at least two or three of these will turn out to be safe and effective. There is no guarantee that any of the vaccine currently in phase 3 trials will turn out to be safe and effective, and most will likely fail. Although it is possible that we will have a safe and effective vaccine by early in 2021, it is very unlikely.

Risks of emergency approval of vaccines

Russia and China have given emergency approval of vaccines that have not yet completed phase 3 trials. This is very dangerous. The bar for safety for giving a vaccine to millions of healthy people is very high. Short cutting the phase 3 trials has the potential to harm thousands of people with a vaccine that we don’t even know will be effective.

Hopefully the U.S. will not follow suit and the FDA will not approve a vaccine until phase 3 trials are completed and a vaccine is shown to be safe and effective.