On 26 January, both Regeneron and Eli Lilly, two key developers of antibody-based interventions for Covid-19, released data showing that their antibody cocktails can reduce the severity of or completely prevent Covid-19 in different settings. These data are both intriguing and highly promising; however, the utility of these agents in controlling the pandemic remains unclear, as their uptake depends on their ability to perform as the data indicates. Passive prophylaxis or passive vaccination, where antibodies are given to a patient to confer protection against a pathogen, is not a new concept. This strategy has been more frequently employed for respiratory syncytial virus (RSV), where an antibody against RSV, palivizumab (Synagis), is prophylactically given to newborns to prevent infection. One reason why this strategy is successful is that no effective vaccine is available. Although passive prophylaxis is used in an exceedingly small cohort of patients, it is considered considerably more expensive than active vaccination would be. For example, a single dose of Synagis in the US costs approximately $1,000, and multiple doses are required. An expensive pediatric vaccine would be approximately one-tenth the cost.

At first glance, the data for both cocktails support their use. In a Phase III study of more than 1,000 high-risk patients recently diagnosed with Covid-19, only 11 patients who received Lilly’s bamlanivimab-etesevimab combo were hospitalised and there were no deaths. In comparison, there were 26 hospitalisations and 10 deaths among placebo patients, translating to a 70% reduction in the risk of hospitalisation or death due to Covid-19. According to data from Regeneron, its antibody cocktail REGEN-COV was 100% effective at preventing symptomatic Covid-19 relative to placebo. Passive prophylaxis with the cocktail reduced the overall rate of infection by half, and all of the infections that did occur were asymptomatic and lasted for less than one week.

With these data strongly supporting their implementation in combatting Covid-19, and both Eli Lilly and Regeneron having Emergency Use Authorisations for antibody-based Covid-19 interventions, it is unclear why they are not seeing robust usage, especially given issues with the vaccine rollout and shortages in the US. The most likely reason for this is that their route of administration is intravenous (IV), which is less than ideal for patients who are in the outpatient setting. Vaccine administration can occur in almost any setting, including a local supermarket while shoppers are active in general vicinity. However, the administration of these antibody cocktails require a visit to a centre capable of IV infusions, which is more troublesome for the patients and presents a potential bottleneck for the caregiver, as only a comparatively small number of patients can receive an IV at the same time compared to a vaccine.

While the IV administration of these agents does complicate the path forward for these agents, it does not represent a complete blockage, as these data strongly support their utility in reducing the number of Covid-19 cases, which is paramount given the state of the outbreak in the US and the current strains to hospital systems.