- Kenny Lin, MD, MPH
Last week, federal health officials announced an ambitious plan to begin free distribution of a vaccine against SARS-CoV-2 within 24 hours of its approval or emergency authorization from the U.S. Food and Drug Administration (FDA). That an efficacious vaccine could, remarkably, become available less than one year after the isolation of the virus that causes COVID-19 is due in part to a public-private program to accelerate vaccine, diagnostic test, and therapy development led by the U.S. Departments of Defense and Health and Human Services known as "Operation Warp Speed." In a New England Journal of Medicine commentary, Drs. Moncef Slaui, Shannon Greene, and Janet Woodcock reviewed the progress of Operation Warp Speed on multiple fronts, including the most promising candidate vaccines in phase 2 and 3 clinical trials. Ultimately, though, the impact of any vaccine on the course of the pandemic in the U.S. and abroad will depend not only on how protective it is against infection and disease transmission, but how the allocation of initially limited supplies is prioritized (e.g., health care and essential workers, vulnerable groups) and what proportion of the population agrees to receive it in the absence of a requirement to do so.
A simulation study in the American Journal of Preventive Medicine estimated the efficacy and percent population coverage that a coronavirus vaccine would need to extinguish the epidemic in the absence of other public health measures such as social distancing and wearing face coverings - in other words, what it would take for a vaccine to allow life to "go back to normal." The effects of a vaccine on productivity losses, hospitalizations, medical costs, and deaths vary depending on what percentage of the population has already been exposed to SARS-CoV-2; one seroprevalence survey from late March to mid-May found a range from 1% to 7% at 10 sites in the U.S., but these figures are likely to be higher four or more months later. To extinguish the epidemic after 5% of the population has been exposed, for example, the AJPM researchers calculated that a vaccine would need to have at least 80% efficacy if administered to 75% of the population.
Unfortunately, expecting any of the coronavirus vaccines in development to have 80% efficacy is unrealistic. By comparison, the Centers for Disease Control and Prevention (CDC) estimated that the 2019-20 influenza vaccine was only 45% effective, consistent with the 40-60% range in previous years when the available vaccines were antigenically matched to circulating influenza viruses. The highest influenza vaccine coverage was during the 2018-19 season, when 63% of children (state range, 46%-81%) and 45% of adults (state range, 34%-56%) received the vaccine. Although some may be more willing to be vaccinated against SARS-CoV-2 than influenza due to the former's greater morbidity and mortality, influenza vaccine also has a long safety track record that a coronavirus vaccine would not.
It is possible that public health measures in place to slow the spread of COVID-19 will substantially reduce the impact of influenza during the 2020-21 season. A CDC surveillance report showed that U.S. cases of laboratory-confirmed influenza fell sharply after the national COVID-19 emergency declaration on March 1, reflecting not only the natural waning of the flu season but also mitigation interventions implemented by states around this time. In the Southern Hemisphere nations of Australia, Chile, and South Africa, where influenza activity normally peaks in June or July, COVID-19 mitigation was associated with a near-complete suppression of influenza circulation.
For family physicians, preparing for flu season during the COVID-19 pandemic, with a coronavirus vaccine potentially around the corner, means adapting to many uncertainties. What hasn't changed from previous years is that the CDC's Advisory Committee on Immunization Practices continues to recommend influenza vaccination for all people six months and older who do not have contraindications, ideally by the end of October. Whenever it arrives, the coronavirus vaccine is unlikely to be a "magic bullet" that ends the pandemic on its own. So it's critical that clinicians all continue to echo the message about cloth face coverings that CDC Director Robert Redfield, MD delivered at a recent Senate hearing: "These face masks are the most important, powerful public health tool we have" for controlling the pandemic.