Two new studies highlight waning mRNA COVID-19 efficacy against infection against the Delta (B1617.2) variant, with one showing that two doses of the Moderna vaccine were highly effective against all variants but that protection against Delta fell over time since vaccination.
The other reveals a precipitous drop in mRNA vaccine effectiveness after the emergence of Delta among an older group of US veterans.
Efficacy against hospitalization stays high
A team led by Kaiser Permanente Southern California scientists evaluated Moderna COVID-19 vaccine effectiveness among 8,153 patients with a positive SARS-CoV-2 test sent for whole-genome sequencing or a negative test from Mar 1 to Jul 27, 2021, before the emergence of the newly recognized, highly transmissible Omicron (B.1.1.529) variant. The study was published today in BMJ.
Among the group, 91.3% were unvaccinated, 1.4% had received one vaccine dose, and 7.3% had gotten two doses. Infected patients were matched in a 1:5 ratio with uninfected controls.
Effectiveness of the two-dose vaccine regimen was 86.7% against infection with the Delta variant, 98.4% against Alpha (B117), 90.4% against Mu (B1621), 96% to 98% against other known variants, and 79.9% against variants in specimens in which genome sequencing failed. Effectiveness against hospitalization with the Delta variant was 97.5%, with no deaths.
One-dose vaccine efficacy against Delta infection was 77.0%. Two doses of the vaccine were less effective among recipients 65 years and older (75.2%) than among those 18 to 64 (87.9%).
In the first 2 months after vaccination, however, efficacy against Delta variant infection fell from 94.1% to 80.0% after 6 months. Declining effectiveness was not as pronounced against non-Delta strains.
Of the 5,186 virus samples sequenced, 39.4% were Delta, 27.7% were Alpha, 11.4% were Epsilon (B1427 and B1429), 6.9% were Gamma (P1), 2.2% were Iota (B1526), 1.4% were Mu, and 11.1% were other variants.
In a Kaiser Permanente news release, senior author Hu Fu Tseng, PhD, MPH, said the study has implications for determining optimal vaccination strategies. "The findings of moderately reduced vaccine effectiveness of Moderna COVID-19 vaccine over time against delta infection supports current booster dose recommendations," he said.
Steep drop in protection in older veterans
Today in JAMA Network Open, Veterans Affairs (VA) and Dartmouth College researchers estimated COVID-19 mRNA vaccine effectiveness against infection in 1,363,180 men 65 years and older enrolled in the Vermont VA system and vaccinated in January and February 2021.
The study spanned January to September 2021, and the investigators divided the data into pre-Delta (January through April), emerging Delta (May and June), and high Delta (July through September) periods. Among all patients, 14,328 tested positive for COVID-19, while 56,952 served as negative controls.
Estimated pre-Delta vaccine effectiveness against SARS-CoV-2 infection with any strain was 94.5% in the first month after the second dose, falling to 87.9% at 3 months. After Delta became dominant, estimated efficacy was 62.0% in the first month, dipping to 57.8% by 3 months, a pattern similar to that before Delta. After 4 months, however, effectiveness fell further, to only 20% by months 5 to 7.
The researchers said that while the study population of older male veterans could limit the findings' generalizability, the rapid decline in COVID-19 vaccine efficacy against infection is notable. "This decrease could be attributable to both the passage of time (ie, a waning effect) and the increased transmission of the Delta variant," they wrote.
