While it’s well established that vaccines against SARS-CoV-2 reduce severe disease and mortality levels, they do not provide complete protection against transmission of COVID-19 infection. A new study sheds further light on the rate of waning of vaccine effectiveness (VE) in preventing infection with, and symptoms of, COVID-19. The study compared results of patients infected with different strains of COVID-19 variants.
A team of scientists at the Bruno Kessler Foundation in Trento, Italy conducted a meta-analysis of 40 studies and found a significant dropoff of protection levels after both an initial vaccine series and a booster shot. The waning was particularly pronounced in the case of infection with the Omicron variant, compared with the previous Delta variant.
The pooled estimates show that 1 month after completing a primary vaccine cycle, VE against symptomatic infection with the Omicron variant was 52.8% (95% confidence interval, 45.3%-60.3%), decreasing to 14.3% (95% CI, 4.4%-24.3%) at 6 months and 8.9% (95% CI, -0.8%-18.6%) at 9 months. One month after receiving a booster shot, VE against symptomatic Omicron infection was 60.4% (CI 95%, 55.5%-65.4%), decreasing to 13.3% (95% CI, 7.2%-19.4%) at 9 months.
VE against laboratory-confirmed infection with Omicron a month after finishing a primary vaccination series was 44.4% (95% CI, 37.7%-51.1%) based on pool estimates. This went down to 20.7% (95% CI, 15.1%-26.4%) at 6 months and 13.4% (95% CI, 7.8%-18.9%) at 9 months. One month after a booster dose, the VE against laboratory-confirmed Omicron infection was at 55.4% (95% CI, 42.4%-68.4%), going down to 36.0% (95% CI, 27.0%-45.0%) at 6 months and 28.9% (95% CI, 17.1%-40.6%) at 9 months.
“Boosters could restore the vaccine protection against symptomatic disease to levels comparable to those estimated soon after completion of the primary cycle,” Piero Poletti, PhD, a researcher at the Bruno Kessler Foundation and an author of the study, told Contagion. “However, for any considered product, VE against Omicron infection and symptomatic disease rapidly wanes over time (VE<40% in 6 months). This means that it is likely that, in most countries, current immunity levels against the infection are driven by immunity acquired by natural infection.”
The scientists found a less pronounced waning dropoff during the Delta variant surge, which occurred before Omicron became the dominant variant. Pooled estimates reveal a VE against symptomatic infection with Delta of 79.6% (95% CI, 72.1%-87.2%) 1 month out from a primary vaccination cycle; 58.5% (95% CI, 48.1%-68.9%) 6 months out; and 49.7% (95% CI, 37.9%-61.5%) 9 months out. The estimated half-life of VE against symptomatic disease was 316 days (95% CI, 240-470 days) for Delta compared with 87 days (95% CI, 67-129 days) for Omicron.
Against confirmed laboratory infection with Delta, VE 1 month from finishing the initial vaccine series was 80.5% (95% CI, 75.3%-85.7%); at 6 months it reached 54.6% (95% CI, 46.5%-62.7%); and at 9 months it was 45.9% (95% CI, 37.5%-54.2%). The estimated half-life of VE against confirmed infection with Delta was 540 days (95% CI, 494-596 days) and 143 days (95% CI, 108-220 days) for Omicron.
Although waning occurs more quickly with Omicron, the addition of a booster dose appears to provide even higher levels of protection than after the initial vaccine series. “From a public health perspective, without additional booster doses we are giving the virus the power of defining its own circulation in the population,” Poletti said. Poletti hopes the study will inform future research into the risk profiles of varying populations with different levels of vaccine uptake.
While the study looked at confirmed infection and symptomatic disease, it did not examine VE against severe disease, hospitalization, and death from COVID-19, which is thought to have a slower rate of decline than VE against infection and symptomatic illness.