In Hungary, the pandemic waves in late 2021 and early 2022 were dominated by the Delta
and Omicron SARS-CoV-2 variants, respectively. Booster vaccines were offered with
one or two doses for the vulnerable population during these periods.The nationwide
HUN-VE 2 study examined the effectiveness of primary immunization, single booster,
and double booster vaccination in the prevention of Covid-19 related mortality during
the Delta and Omicron waves, compared to an unvaccinated control population without
prior SARS-CoV-2 infection during the same study periods. The risk of Covid-19 related
death was 55% lower during the Omicron vs. Delta wave in the whole study population
(n=9,569,648 and n=9,581,927, respectively; rate ratio [RR]: 0.45, 95% confidence
interval [CI]: 0.44-0.48). During the Delta wave, the risk of Covid-19 related death
was 74% lower in the primary immunized population (RR: 0.26; 95% CI: 0.25-0.28) and
96% lower in the booster immunized population (RR: 0.04; 95% CI: 0.04-0.05), vs. the
unvaccinated control group. During the Omicron wave, the risk of Covid-19 related
death was 40% lower in the primary immunized population (RR: 0.60; 95% CI: 0.55-0.65)
and 82% lower in the booster immunized population (RR: 0.18; 95% CI: 0.16-0.2) vs.
the unvaccinated control group. The double booster immunized population had a 93%
lower risk of Covid-19 related death compared to those with only one booster dose
(RR: 0.07; 95% CI. 0.01-0.46). The benefit of the second booster was slightly more
pronounced in older age groups.The HUN-VE 2 study demonstrated the significantly lower
risk of Covid-19 related mortality associated with the Omicron vs. Delta variant and
confirmed the benefit of single and double booster vaccination against Covid-19 related
death. Furthermore, the results showed the additional benefit of a second booster
dose in terms of SARS-CoV-2 infection and Covid-19 related mortality.
