Between 2019 and 2023, the COVID-19 pandemic caused 760 million infections and nearly 7 million deaths, presenting the largest global emergency in modern history (1). After years of enforcing isolation regulations, event restrictions, and social distancing protocols, the United States government lifted the public health emergency on April 11, 2023, forty months after the initial outbreak in Wuhan, China (2). Although the Centers for Disease Control (CDC) no longer perceives COVID-19 as a serious threat due to the introduction of vaccines and the significant reduction in cases, the disease continues to circulate (3). As a result, scientists and public health officials are considering a COVID booster for the fall.
During the pandemic, several variants emerged from the ancestral strain of the SARS-CoV-2 virus, each one exhibiting different levels of transmissibility (4). The first vaccines approved by the Federal Drug Administration (FDA), manufactured by Moderna and Pfizer-BioNTech, respectively, targeted the initial strain of the virus, which helped neutralize infections caused by all strains, but with varying efficacy (5). After studies showed that some variants, specifically the Delta and Omicron strains, could evade vaccine-induced immune responses, scientists realized the need for a different approach, leading to manufacturers releasing bivalent vaccines that contained mRNA components of the initial variant and the BA.4 and BA.5 Omicron variants in August 2022 (6). Compared to the original vaccines, this strain-specific strategy appeared to provide better protection against severe COVID-19 symptoms from more strains, creating an innovative precedent for future formulations (7). After months of reviewing research and World Health Organization (WHO) recommendations, FDA advisors voted unanimously to target the XBB strain, a derivative of Omicron, for a booster in the fall to continue fighting against COVID-19 (8).
Similar to the way in which the BA.4 and BA.5 variants were selected for the bivalent vaccine, the XBB.1.5 strain was chosen as the target for the newest booster due to the elevated risk it poses to Americans. Currently, according to the CDC, strains from this lineage constitute 95% of all circulating variants (9). Federal public health officials predict that the sublineage XBB.1.16 will become the dominant COVID-19 strain in the fall, while other XBB sublineages will also increase in prevalence (10). Thus, targeting XBB.1.5 will provide specific protection against this strain, as well as other variants in the XBB lineage (9). In combination with the previous monovalent vaccines and the bivalent booster, the fall booster will provide additional protection against COVID-19, with bolstered defense against this emerging strain.
Although FDA advisors agreed about the necessity of the XBB booster, the experts have not reached consensus on the target population (10). Vulnerable populations, such as elderly adults and individuals with immunocompromising conditions, will likely derive the most benefit from an updated fall booster, as this population gained increased protection against severe COVID-19 symptoms following the introduction of the bivalent booster (11). Although the bivalent booster showed 62% efficacy in preventing hospitalization for up to two months following vaccine administration, this rate drops to 24% by four to six months (12). Additionally, protection against intensive care unit (ICU) admission and death decreases from 69% to 50% in the same time frame (12). Thus, there is a significant decline in COVID-19 protection for vulnerable populations, indicating a need for the XBB vaccine to close the immunity gap and reduce risk of contracting the most common variants. For healthy individuals, however, the necessity for the XBB booster is unclear (10). After the approval of the bivalent boosters, experts argued that healthier, younger individuals are more able to fight off COVID-19 infection and avoid severe symptoms, so receiving boosters is unnecessary (13). Additionally, the WHO recommended that healthy, young individuals did not need the bivalent booster, while older and immunocompromised individuals should receive it (14). In the coming months, after manufacturers develop the XBB boosters, the FDA is expected to issue guidance regarding the target population for the booster, though experts predict that only high-risk individuals will be eligible for this vaccine (13).
References
1: World Health Organization. 2023. Coronavirus (COVID-19) dashboard. WHO. URL: https://covid19.who.int/.
2: Miller, Z. 2023. Biden ends COVID national emergency after Congress acts. AP News. URL: https://apnews.com/article/joe-biden-covid19-coronavirus-national-emergency-e3a52722b57a6b4f24187426c27b3b39.
3: Centers for Disease Control and Prevention. 2023. Trends in United States COVID-19 hospitalizations, deaths, emergency visits, and test positivity by geographic area. CDC COVID Data Tracker. URL: https://covid.cdc.gov/covid-data-tracker/#trends_weeklyhospitaladmissions_select_00.
4: El-Shabasy, R., Nayel, M., Taher, M., Abdelmonem, R., Shoueir, K., and Kenawyg, E. 2022. Three waves changes, new variant strains, and vaccination effect against COVID-19 pandemic. International Journal of Biological Macromolecules, vol. 204. DOI: 10.1016/j.ijbiomac.2022.01.118.
5: Zeng, B., Gao, L., Zhou, Q., Yu, K., and Sun, F. 2022. Effectiveness of COVID-19 vaccines against SARS-CoV-2 variants of concern: a systematic review and meta-analysis. BMC Medicine, vol. 20. DOI: 10.1186/s12916-022-02397-y.
6: Food and Drug Administration. 2022. Coronavirus (COVID-19) update: FDA authorizes Moderna, Pfizer-BioNTech bivalent COVID-19 vaccines for use as a booster dose. FDA News Release. URL: https://www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-fda-authorizes-moderna-pfizer-biontech-bivalent-covid-19-vaccines-use.
7: Shrestha, N., Burke, P., Nowacki, A., Simon, J., Hagen, A., and Gordon, S. 2023. Effectiveness of the coronavirus disease 2019 bivalent vaccine. Open Forum Infectious Diseases, vol. 10. DOI: 10.1093/ofid/ofad209.
8: Food and Drug Administration. 2023. Updated COVID-19 vaccines for use in the United States beginning in fall 2023. FDA. URL: https://www.fda.gov/vaccines-blood-biologics/updated-covid-19-vaccines-use-united-states-beginning-fall-2023.
9: Centers for Disease Control and Prevention. 2023. Summary of variant surveillance. CDC COVID Data Tracker. URL: https://covid.cdc.gov/covid-data-tracker/#variant-summary.
10: Constantino, A. 2023. FDA recommends that updated COVID shots target Omicron subvariant XBB.1.5 this fall. CNBC. URL: https://www.cnbc.com/2023/06/16/fda-covid-shots-should-target-omicron-subvariant-xbbpoint1point5-this-fall.html.
11: Lontok, K. 2023. How effective are COVID-19 vaccines in immunocompromised people? American Society for Microbiology. URL: https://asm.org/Articles/2021/August/How-Effective-Are-COVID-19-Vaccines-in-Immunocompr.
12: Wen, L. 2023. A new, sensible plan for fall COVID boosters is taking shape. The Washington Post. URL: https://www.washingtonpost.com/opinions/2023/06/27/covid-updated-booster-vaccine-xbb-fall/.
13: Smith, D. 2023. Who should get a COVID booster now? New data offers some clarity. The New York Times. URL: https://www.nytimes.com/2023/02/02/well/live/covid-bivalent-booster-omicron.html.
14: World Health Organization. 2023. SAGE updates COVID-19 vaccination guidance. WHO. URL: https://www.who.int/news/item/28-03-2023-sage-updates-covid-19-vaccination-guidance.