Apr 13th, 2022
The omicron variant is genetically divergent from the ancestral SARS-CoV-2 strain for which the current vaccines were made.
In a study from Israel that was recently published in the NEJM, two distinct groups of patients were given a fourth dose of the Pfizer vaccine. One group included those over 60 years of age, as well as individuals under 60 with risk factors that made them more susceptible to severe disease. The second group included health care workers. The fourth dose was administered four months after receiving the third dose. Patients previously infected were excluded. The study included more than 1.2 million people and the investigators reviewed data for 4-6 weeks post vaccination, a very short time period indeed.
Results: The fourth dose of the vaccine reduced the incidence of confirmed infection by half in the first four weeks, but by week eight, there was no difference between the rate of infection for those given the fourth booster and those without. The rate of severe illness was also diminished in the group receiving the fourth vaccination as compared to those receiving only three vaccinations. However, the absolute value of this difference was very small. The benefit was only 1-2 per 100000 person days but was still present by week six when the study ended. It is important to note that there was no substantial risk of receiving a fourth dose of this vaccine.
Another open-label, nonrandomized clinical study that compared the efficacy of the fourth dose of the Pfizer and Moderna vaccine in young healthy health care workers, showed that these vaccines were only marginally beneficial, even though maximal immunogenicity (immune response) was restored again with the fourth dose just as it was temporarily with the third dose. Those infected were less symptomatic as opposed to the control group, but carried a high viral load and were infectious, nevertheless.
The questions we need to therefore ask ourselves:
- When should one receive the booster in relation to the transmission in one’s community?
Unfortunately, the answer is not that simple. Now would be as good a time as any if you are in a high-risk group. - How do vaccines work in the face of a changing pathogen?
Repetitive use of the vaccines based on the original ancestral strain of the virus (alpha, beta and delta) have encouraged a viral evolution in a direction that escapes prior established immunity. We need vaccines that will decrease transmission which the current vaccines do not do for the Omicron strain. This is in contrast to the very beginning in which the vaccines were highly effective in reducing transmission. - And last but not the least, the study showed diminishing returns to using the same vaccines repeatedly. The beta and delta-based vaccines have less clinical and public health value today given the dominance of Omicron.
Take home message: The current covid vaccines do not prevent new infections and even though serious illness is curtailed, the benefits only last weeks. Ultimately until a better, more relevant vaccine for the current strain, and newer and more specific vaccines for future evolved strains are produced, this saga may continue. Till then wearing a mask may not be such a bad idea after all.
