The few adults who receive a COVID-19 mRNA vaccine but still become infected have a milder, shorter illness and lower viral RNA loads than their unvaccinated peers, finds a real-world US study yesterday in the New England Journal of Medicine.
The journal also features a UK study confirming that the Novavax vaccine offers 90% overall effectiveness and protects against the highly transmissible and potentially more deadly Alpha (B117) SARS-CoV-2 variant.
mRNA vaccines 91% effective after 2 doses
In the first study, a team led by the Centers for Disease Control and Prevention COVID-19 Response Team tested 3,975 coronavirus-naïve healthcare workers, first responders, and other essential workers who received one or two doses of the Pfizer/BioNTech or Moderna mRNA vaccine or were unvaccinated from Dec 14, 2020, to Apr 10, 2021. The study took place in Arizona, Florida, Minnesota, Oregon, Texas, and Utah.
Of the participants, 3,179 (80%) received at least one dose by the end of the data-collection period, while 2,686 (84%) were given two doses. Of the vaccines, 67% were Pfizer and 33% were Moderna, while the brand name wasn't recorded in less than 1%.
In total, 204 (5%) tested positive for COVID-19, of whom 5 were fully vaccinated, 11 were partially vaccinated, and 156 were unvaccinated. The 32 remaining participants had an undetermined vaccination status and were excluded from the analysis.
In fully vaccinated participants, adjusted vaccine effectiveness was 91% (95% confidence interval [CI], 76% to 97%), falling to 81% (95% CI, 64% to 90%) in the partially vaccinated group.
Fully or partially vaccinated participants had a 40% lower average viral RNA load (95% CI, 16% to 57%) than the unvaccinated group, a 58% lower risk of fever (relative risk, 0.42; 95% CI, 01.8 to 0.98), and a shorter illness (-6 days of symptoms, -2 days spent sick in bed; 95% CI, 0.8 to 3.7).
Three quarters of those who received at least one vaccine dose had detectable viral RNA loads for 1 week; whereas, detectable RNA lingered longer in 72% of the unvaccinated cohort. The researchers say this means partial or full vaccination lowers the risk for viral RNA detection longer than 1 week by 66%.
"If you get vaccinated, about 90% of the time you're not going to get COVID-19," study coauthor Jeff Burgess, MD, MPH, said in a University of Arizona Health Sciences news release. "Even if you do get it, there will be less of the virus in you and your illness is likely to be much milder."
Implications for frontline workers
The genomes of 93 viruses were sequenced, revealing 11 coronavirus variants of concern or interest (Epsilon [B1427 and B1429]), 9; Alpha, 1; and Zeta [P2], 1). Ten such variants were identified among partially or fully vaccinated participants, three of them (30%) Epsilon, compared with 7 of the 70 viruses (10%) found in the unvaccinated group.
Among the coronavirus-infected participants, 26% required medical attention, including three unvaccinated participants who were admitted to the hospital. No deaths were reported.
Most study participants were women (62%), aged 18 to 49 years (72%), White (86%), and non-Hispanic (83%), with no underlying medical conditions (69%).
If the study's results are confirmed after further data collection, the researchers wrote, "the overall results [would] support that mRNA vaccines not only are highly effective in preventing SARS-CoV-2 infection but also may mitigate the effects of breakthrough infections—a finding that is especially important to essential and frontline workers, given their potential to transmit the virus through frequent close contact with patients, coworkers, and the public."
Novavax 86.3% effective against Alpha
The second study, led by researchers from the Vaccine Institute at the University of London, was part of an ongoing phase 3 trial of the Novavax recombinant nanoparticle COVID-19 vaccine. The study involved 14,039 participants aged 18 to 84 years at 33 sites in the United Kingdom from Sep 28 to Nov 28, 2020. The newly released results come on the heels of the company's Jun 14 announcement of 90% overall vaccine efficacy.
Participants were assigned to receive two doses of the Novavax vaccine or a placebo in a 1:1 ratio 21 days apart. Of all participants, 27.9% were 65 or older, 48.4% were women, 94.5% were White, 2.9% were Asian, 0.4% were Black, and 44.6% had a chronic illness.
Ten vaccinees and 96 placebo recipients tested positive for COVID-19, with symptom onset 7 or more days after the second injection, for an 89.7% vaccine efficacy (95% CI, 80.2% to 94.6%).
In a post hoc analysis, the vaccine showed 86.3% (95% CI, 71.3% to 93.5%) efficacy against symptomatic infections caused by the Alpha variant and 96.4% (95% CI, 73.8% to 99.5%) efficacy against symptomatic infections from other strains.
Solicited adverse local and systemic reactions, while more common in the vaccine group than in placebo recipients, were mainly mild and lasted no more than 2 or 3 days; few serious adverse events (0.5%) occurred in either group. Two deaths occurred, one in the vaccine group and one in a placebo recipient, both of whom died of COVID-19 complications.
The researchers said that although their study was not powered to evaluate vaccine efficacy against all SARS-CoV-2 strains, the results are reassuring. "In particular, the efficacy estimate of 96.4% against the non-B.1.1.7 strains (the majority of which were the prototype strain) is similar to the efficacy of 95.0% reported against this strain for the [Pfizer vaccine] and the efficacy of 94.1% for the [Moderna vaccine]," they wrote.
