A week ago, Pfizer announced that its COVID-19 was 90% effective. This morning, Moderna has announced that its vaccine is almost 95% effective. These are promising findings in the public health fight against the COVID-19 pandemic, but public health authorities are warning that this is not the beginning of the end for the disease. There are still more studies to be done on vaccine safety, and there is much more planning that needs to be sorted out so the vaccine can be distributed as equitably and effectively as possible.
So what is vaccine effectiveness?
Vaccine effectiveness is the reduction in disease incidence between vaccinated and non-vaccinated groups in real-life settings. This is different that vaccine efficacy, which is a similar concept but one that is analyzed in clinical trial settings under controlled conditions. What both vaccine manufacturers in this case did was look at people who had tested positive for COVID-19 and then unblind the vaccine status (vaccine or placebo) of those people to see how many of them had been vaccinated. As it turns out, between 90% and 95% of those who contracted the disease had received a placebo and not the vaccine. In other words, if you were a research participant who received the vaccine, your chance of being in the group that contracted COVID-19 was about 5%. If were a research participant who did not receive the vaccine — but received a placebo — your chance of being in the group that contracted COVID-19 was about 95%.
This high effectiveness is a good sign that robust community immunity (herd immunity). For comparison, the influenza vaccine has a much lower effectiveness year to year. However, much like the influenza vaccine — and other vaccines — the new COVID-19 vaccines might have varied effectiveness depending on who gets them. For example, we know that older adults and people with immune-suppressive conditions (or taking medication for autoimmune disorders) may need more than one vaccine dose to receive the same protection as someone with a regular immune system. We also know that risk of contracting COVID-19 is higher in some groups because of their social and economic characteristics. All of these factors will need to be weighed for the vaccines to have the most impact in stopping the pandemic.