Yesterday there were a number of news stories highlighting Dr. Anthony Fauci’s belief that the chances of having a “highly effective” coronavirus COVID-19 vaccine were “slim.” The statement given during a session at the Brown University School of Public Health drew much attention, but the actual meaning of his statement needs clarification.
Dr. Fauci defined a highly effective vaccine as one that provides 98% or more guaranteed protection. That is an incredibly high bar as most vaccines don’t reach that level of protection. The current, most effective vaccines for preventing viral infections, Gardasil for cervical cancer (human papilloma virus) and Shingrix for Shingles (varicella virus) are 100% and 95% effective respectively. Even the measles vaccine is 90% effective. So, you don’t need a 98% guarantee to have an effective vaccine!
On the other hand, what might be an acceptable level of coverage? People point to the influenza vaccine, which ranges from 50-70% effectiveness dependent on the year. This is because the influenza strain changes every year, therefore we basically have to guess what to put in the vaccine. The bigger problem is that the flu viruses change every year, so you need a new vaccine every year. There hasn’t yet been evidence of this with SARS-CoV-2, which causes COVID-19.
While the FDA and vaccine experts are hoping for a coronavirus vaccine that is at least 75% effective, they have said 50% or 60% effective would be acceptable. Dr. Stephen Hahn, the FDA’s commissioner, said last month that “the vaccine or vaccines that end up getting authorized will prove to be more than 50% effective, but it’s possible the U.S. could end up with a vaccine that, on average, reduces a person’s risk of a Covid-19 infection by just 50%.”
Would such a vaccine be useful? The answer is a resounding “maybe.”
If the immunity really protects 50% of the people who receive the vaccine and is relatively long lasting, let’s say 5 years, it could help to develop herd immunity and suppress the virus to very low levels so new cases are minimized. It could also provide additional time for the development of a more effective and longer lasting vaccine.
On the other hand, if the immunity is marginal, barely protecting 50% of those immunized and does not last very long, it would be of little value. It would be particularly problematic if it did not work well in the elderly who need it most. These problems would be magnified if the vaccine had significant side effects that kept people from agreeing to be vaccinated.
The COVID-19 vaccines will get quite a bit of scrutiny when they are released, and that scrutiny will be warranted.