Understanding the Booster Shot Landscape

In anticipation of declining immunity against SARS-CoV-2, health authorities worldwide are preparing for the rollout of new booster shots in the near future. However, the situation has evolved significantly. With the global emergency status of COVID-19 lifted and a notable decrease in cases, it is crucial to reassess the criteria for booster eligibility.

Some countries have opted to restrict the distribution of booster shots to those who are at the highest risk of severe illness or death. This trend is expected to continue with the upcoming booster campaigns, focusing on safeguarding the most vulnerable populations.

Conversely, there are nations advocating for a broader vaccination initiative that encompasses all age groups. A number of scientists support this approach, arguing that widespread vaccination can provide indirect protection to at-risk individuals.

Since late 2022, many countries have introduced bivalent boosters aimed at both the original SARS-CoV-2 strain and a preceding Omicron variant. While these boosters offer a certain level of protection, their effectiveness wanes rapidly, as indicated by CDC data showing a drop in hospitalization protection from 62% to 24% within a mere four months.

In 2023, the US Food and Drug Administration's (FDA) advisory committee endorsed an update to the COVID-19 vaccine to target the singular XBB strain. This forthcoming monovalent vaccine is anticipated to better match the currently circulating strain during its distribution.

Emerging data for the XBB-targeted vaccines implies that including the original SARS-CoV-2 strain may not enhance the immune response to circulating subvariants. This finding influenced the World Health Organization’s (WHO) Technical Advisory Group’s recommendation in May 2023, which favored the shift from vaccines containing the original strain to those that promote neutralizing antibodies against XBB subvariants.

As a result, vaccine manufacturers like Moderna, Novavax, and Pfizer are now focusing their efforts on the XBB.1.5 variant, assuring that these updated vaccines will be ready for public use by late summer or early autumn.

Despite this progress, public interest in vaccination is waning as immunity diminishes. The uptake for each successive booster shot has decreased, with only around 17% of the US population and approximately 14% of Europeans having received a second booster dose.

The WHO continues to recommend routine booster shots for high-risk groups, including the elderly and healthcare professionals. However, it does not advocate routine boosters for healthy adults under 60 who have completed their initial vaccination series, expressing hesitancy regarding COVID-19 vaccines for healthy children as well.

In line with this guidance, countries like the UK have already adjusted their booster strategies. As of February, officials stopped administering boosters to healthy individuals under 50. Many European nations are likely to adopt similar policies for the upcoming monovalent boosters.

In contrast, the US and Japan maintain a more expansive booster strategy, offering shots to children aged six months and older. The CDC is still deliberating on whether to recommend the monovalent vaccine for all age groups. Meanwhile, Japan plans to provide another booster for healthy individuals aged five and up between September and December, possibly on an annual basis.

While the WHO suggests lower vaccination priority for healthy children and young adults, the strategies of the US and Japan can be justified, as these vaccines do offer benefits, albeit temporary. Vaccinating healthy individuals also contributes to herd immunity, thereby indirectly protecting at-risk populations.

However, navigating the evolving vaccine landscape will become increasingly intricate, even for older adults. Following the recent FDA approval of the first two RSV vaccines for older individuals, they now have the option to receive shots against respiratory syncytial virus (RSV) alongside the new COVID-19 vaccines.

This situation presents a significant challenge, particularly due to the limited data available on the co-administration of the RSV vaccine with other recommended vaccines, including the COVID-19 shots.

The CDC’s Advisory Committee on Immunization Practices (ACIP) refrained from making a blanket recommendation for all individuals aged 60 and above to receive the RSV shot this fall, citing the need for further research on its compatibility with other vaccines.

During the ACIP meeting in June 2023, medical experts noted that clinical trial data are only available concerning the simultaneous administration of the RSV and flu vaccines. Unfortunately, we still lack comprehensive information on how the RSV vaccine interacts with other vaccines recommended for the elderly, such as those for COVID-19.