Yesterday, AstraZeneca announced its intent to seek emergency use authorization from the U.S. Food and Drug Administration for its COVID-19 vaccine. This was based upon their conclusions form the U.S. Phase III trial that demonstrated statistically significant vaccine efficacy of 79% at preventing symptomatic COVID-19 and 100% efficacy at preventing severe disease and hospitalization. However, hours later, the National Institute of Allergy and Infectious Disease was notified by the Data and Safety Monitoring Board of “outdated information from that trial, which may have provided an incomplete view of the efficacy data.” NIAID is urging AstraZeneca to work with the DSMB to update the efficacy data and make it publicly available as soon as possible.
- Disease Management
Herd Immunity Or A New Normal?
A new debate is emerging in the scientific community on whether herd immunity against COVID-19 is achievable, or whether a new normal of living with the virus is more likely. Herd immunity refers to the concept in virology where the portion of the population having protection — induced from vaccination or previous infection — is large enough to prevent transmission of the disease. Unfortunately, the novel SARS-CoV-2 coronavirus and its vaccines are so novel that data supporting the possibility of herd immunity are clouded in uncertainty. For starters, estimates on the portion of the population needing protection range widely from 60% to 85%. Furthermore, due to large rates of asymptomatic cases, no one knows what the actual infection rate is, although the Centers for Disease Control and Prevention estimates it to be 4.6 times the known infection rate in the U.S. Additionally, the duration of seropositive protection is unknown, as is the efficacy of the vaccines against emerging variants. Other significant barriers underly the fact that the vaccines are not currently approved for children who make up one-fifth of the population and unanticipated sources of vaccine hesitancy — particularly along political lines — are becoming manifest.
Considering these unknowns, it is important to emphasize the words “crude estimates” in the following analysis designed to project possible dates when the critical mass of Missourians with either form of protection will converge with estimates required to induce herd immunity. Inputs include known rates of infection, CDC estimates of latent infections and known rates of vaccine series completion by Missouri residents. By March 21, 9.3% of Missouri’s total population had tested positive with diagnostic or antigen tests — using CDC estimates on unreported cases suggests the true disease burden to be 42.8% of Missourians with potential seropositive protection. On the same date, 690,000 Missourians (11.2% of the population) had completed the vaccine series. Assuming the same portion of the population with protection from a previous infection sought out the vaccines infers that 6.4% of the population has vaccine-induced protection without seropositivity from a previous infection. Using the average daily percent change in known infections and vaccine series completion between Feb. 1 and Mar. 21, 2021, to impose linear projections on the data suggests that 65% of Missourians will have some form of protection against COVID-19 by July 6. Following recent linear trends, the analysis suggests the mid-level estimate required for herd immunity of 75% would be reached by Sept. 22, and 85% of the population would have some form of protection by Dec. 22. While it is important to have targeted dates to anticipate, it also is important to understand that any one of the myriad assumptions underlying this analysis are subject to change — for example, another surge in cases, or vaccine demand becoming more anemic would significantly alter convergence dates.
MHA and HIDI will continue to monitor the data and update these projections and underlying modeling accordingly. It is critically important that Missourians continue to stave off complacency with continued masking, physical distancing and hand hygiene in addition to becoming vaccinated when eligible.