The rapid development and deployment of COVID-19 vaccines were crucial in combating the pandemic. In Europe, four vaccines—BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), ChAdOx1 (AstraZeneca), and Janssen—received authorization. These vaccines utilize the SARS-CoV-2 spike protein as an antigen. Early vaccination strategies in Finland employed a 3-week interval between doses of BNT162b2, later extended to 12 weeks. The use of ChAdOx1 was restricted due to reports of blood clotting events, leading to heterologous vaccination strategies (ChAdOx1 followed by mRNA vaccines). Studies indicated that heterologous vaccination might enhance immune responses. However, antibody levels wane over time, highlighting the need for booster doses, especially with the emergence of variants of concern (VOCs) like Delta and Omicron. This study aimed to compare the antibody responses and neutralizing antibody titers against different SARS-CoV-2 variants following various vaccination regimens, focusing on the impact of dose interval and booster shots.

Existing literature highlighted the varying efficacy and immune responses generated by different COVID-19 vaccines and vaccination schedules. Studies showed that heterologous vaccination strategies, particularly using ChAdOx1 followed by an mRNA vaccine, could lead to higher neutralizing antibody levels compared to homologous strategies. The waning of immunity over time and the emergence of VOCs, such as Delta and Omicron, raised concerns about the long-term effectiveness of initial vaccination strategies. Previous research also demonstrated reduced neutralizing antibody activity against Omicron compared to earlier variants, underscoring the need for booster doses to maintain protection against evolving SARS-CoV-2 strains. The literature also explored the impact of different dosing intervals, with some studies suggesting that longer intervals might lead to more robust and sustained immune responses.

This study enrolled 328 healthcare workers (HCWs) who received two doses of COVID-19 vaccines with either a short (3-week) or long (12-week) interval. The vaccines included BNT162b2, mRNA-1273, ChAdOx1, and combinations thereof. A subset of HCWs (n=120) who received two doses of BNT162b2 with a 3-week interval also received a third booster dose (BNT162b2 or mRNA-1273). Sequential serum samples were collected to measure SARS-CoV-2 S1-specific IgG antibody levels using enzyme immunoassay (EIA). Microneutralization tests (MNT) assessed the neutralizing capacity of sera against D614G (ancestral), Delta, and Omicron BA.1 variants. Demographic data were collected, and statistical analyses were performed to compare antibody responses and neutralization titers among different groups. The study adhered to ethical guidelines, obtaining written informed consent from all participants.

The study found that two doses of BNT162b2, mRNA-1273, or a combination of ChAdOx1 and an mRNA vaccine (with a 12-week interval) elicited high levels of anti-SARS-CoV-2 spike antibodies and neutralizing antibodies against the D614G and Delta variants. However, a shorter 3-week interval between two BNT162b2 doses resulted in 2-3-fold lower neutralizing antibody titers. Importantly, a third booster dose of BNT162b2 or mRNA-1273 significantly increased antibody levels (4-fold compared to after the second dose) and induced neutralizing antibodies against the Omicron BA.1 variant. The neutralizing antibody titers against D614G, Delta, and Omicron BA.1 variants decreased over time after the second dose, but the third booster dose restored neutralizing activity against all three variants. mRNA-1273 generally elicited slightly higher antibody responses compared to BNT162b2. The study also found that the length of the dosing interval significantly impacted antibody levels, with longer intervals leading to higher and more sustained responses. Finally, the rate of breakthrough infections was very low after two vaccine doses, regardless of the interval between doses.

This study's findings demonstrate that while two doses of various COVID-19 vaccines provide significant protection against earlier variants like Delta, a third booster dose is crucial for broadening protection against emerging VOCs like Omicron. The observed waning of neutralizing antibodies against Omicron after two doses underscores the need for booster strategies. The higher antibody responses observed with mRNA-1273 compared to BNT162b2 aligns with previous findings, suggesting potential differences in vaccine formulations. The impact of the dosing interval emphasizes the importance of optimal vaccination schedules. These results have critical implications for public health strategies, highlighting the need for continued monitoring of vaccine effectiveness against emerging variants and adapting vaccination strategies accordingly. The low rate of breakthrough infections underscores the overall effectiveness of the vaccination programs.

This study demonstrates the importance of booster doses in enhancing the protection provided by COVID-19 vaccines against evolving SARS-CoV-2 variants. The findings highlight the superior immunogenicity of mRNA-1273 and the significant impact of longer dosing intervals on antibody responses. The results support current recommendations for booster shots to maintain high levels of neutralizing antibodies against multiple variants, especially newer ones like Omicron. Further research could explore the duration of protection provided by the third dose and investigate the role of cellular immune responses in long-term immunity.

The study was conducted on healthcare workers, which may limit the generalizability of the findings to the broader population. The follow-up period after the third dose was relatively short, and longer-term observation is needed to fully assess the durability of protection. The study focused primarily on humoral immunity, while the contribution of cell-mediated immunity should be further explored. The specific Omicron subvariant analyzed (BA.1) may not fully represent the spectrum of circulating Omicron variants.