Vaccine-induced immunity
The short-term (<4 months) efficacy of mRNA vaccines against SARS-CoV-2 is high and varies from 94.1% (Moderna) to 95% (BioNTech/Pfizer). This has been confirmed by randomized controlled trials and was subsequently confirmed in effectiveness studies in real-world settings. Waning efficacy was observed with respect to protection against SARS-CoV-2 infections (for example, only approximately 20% after about half a year in Qatar), whereas protection against severe disease was either sustained or showed only a moderate decline.
In individuals who received two doses of the BioNTech/Pfizer vaccine at least 5 months earlier, an additional vaccine dose, a so-called booster, significantly lowered mortality and severe illness. These findings suggest that the booster restored and probably exceeded the initial short-term efficacy of the initial vaccination.
Data are still emerging regarding the efficacy of boosters against the Omicron variants. Preliminary data suggest a far lower ability to restore protection from infection and vaccination. However, fatalities and hospitalizations remain low.
Natural immunity vs. vaccine-induced immunity
Comparisons of natural immunity with vaccine-induced immunity are complicated by a series of biases and by combinations of biases – for example, the biases of comparisons between infected and uninfected, plus the biases of comparisons between vaccinated and nonvaccinated, with strong potential selection biases and confounding. Of particular note, the proportion of people previously infected and/or vaccinated may influence estimates of effectiveness. Regarding this point, one study compared unvaccinated patients with a prior SARS-CoV-2 infection and vaccinated individuals followed up from a week after the second vaccine dose onward versus a group of unvaccinated, not previously infected individuals. The findings showed that, compared with unvaccinated, not previously infected individuals, the natural immunity group and the vaccinated group had similar protection of 94.8% and 92.8% against infection, of 94.1% and 94.2% against hospitalization, and of 96.4% and 94.4% against severe illness, respectively.
Hybrid immunity
The combination of a previous SARS-CoV-2 infection and a respective vaccination is called hybrid immunity. This combination seems to confer the greatest protection against SARS-CoV-2 infections, but several knowledge gaps remain regarding this issue.
Data from Israel showed that, when the time since the last immunity-conferring event (either primary infection or vaccination) was the same, the rates of SARS-CoV-2 infections were similar in the following groups: individuals who had a previous infection and no vaccination, individuals who had an infection and were then vaccinated with a single dose after at least 3 months, and individuals who were vaccinated (two doses) and then infected. Severe disease was relatively rare overall.
Data on the efficacy of hybrid immunity point in the direction of hybrid immunity being superior, as compared with either vaccine-induced (without a booster) immunity or natural immunity alone. Timing and mode of vaccination of previously infected individuals to achieve optimal hybrid immunity are central questions that remain to be addressed in future studies.
Given that vaccination rates are continuously increasing and that, by the beginning of 2022, perhaps half or more of the global population had already been infected with SARS-CoV-2, with the vast majority of this group not being officially detected, it would appear logical that future infection waves, even with highly transmissible variants of SARS-CoV-2, may be limited with respect to their maximum potential health burden. The advent of Omicron suggests that massive surges can occur even in populations with extremely high rates of previous vaccination and variable rates of prior infections. However, even then, the accompanying burden of hospitalizations and deaths is far less than what was seen in 2020 and 2021. One may argue that the pandemic has already transitioned to the endemic phase and that Omicron is an endemic wave occurring in the setting of already widespread population immunity.
A version of this article first appeared on Medscape.com.