Several randomized placebo-controlled Phase III trials have shown that messenger RNA (mRNA) vaccines for COVID-19 effectively prevent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. However, the effectiveness of these vaccines in preventing asymptomatic and symptomatic COVID-19 infections is not well understood.
A new Morbidity and Mortality Weekly Report published by the U.S. Centers for Disease Control and Prevention (CDC) states that mRNA COVID-19 vaccines are highly effective in preventing SARS-CoV-2 infection in the real world. The report is based on a CDC study where it routinely tested prospective cohorts of health care professionals, first responders, and essential and frontline workers for SARS-CoV-2 infections.
The participants were tested weekly in eight different U.S. locations between December 14, 2020, and March 13, 2021. A total of 3,950 individuals were tested for 13 consecutive weeks regardless of symptom status and at the onset of symptoms associated with COVID-19–like illness. The eight U.S. locations were Phoenix, Tucson, and other areas in Arizona; Duluth, Minnesota; Miami, Florida; Temple, Texas; Portland, Oregon; and Salt Lake City, Utah. Over 50% of the participants were from Arizona and included physicians, primary health care personnel, nurses, and other allied health care personnel, first responders, and other essential and frontline workers.
Over 60% of the participants were female, and over 70% were aged 18–49 years. Nearly 70% of the cohort had no chronic medical conditions. The participants had no previous documented SARS-CoV-2 infection. Nearly 63% (2,479) of the participants received both doses of mRNA vaccines and 12% (477) received only the first dose of mRNA vaccine.
In unvaccinated participants, 1.38 SARS-CoV-2 infections per 1,000 person-days were confirmed by reverse transcription-polymerase chain reaction (RT-PCR). In contrast, among the fully immunized individuals tested 14 days after the second dose of the vaccine, 0.04 infections were reported per 1,000 person-days. In the partially immunized individuals tested 14 days after the first dose and before the second dose of the vaccine, 0.19 infections were detected per 1,000 person-days.
Results show 90% and 80% effectiveness of mRNA vaccines in preventing SARS-CoV-2 infections in fully and partially immunized individuals, respectively
The study results showed that the mRNA vaccine's effectiveness for the prevention of SARS-CoV-2 infection was 90% under real-world conditions regardless of the status of symptoms and that for partially immunized individuals was 80%. These findings have significant implications in public health as they show that authorized mRNA vaccines for COVID-19 are highly effective in preventing infections by SARS-CoV-2 in real-world conditions.
The findings of the CDC study complement and build upon preceding reports from studies by demonstrating that the mRNA vaccines are also effective in reducing the infection risk regardless of COVID-19–associated symptom status. Reducing the risk for transmission, which is possible with asymptomatic infection or days before symptoms onset, is crucial among first responders, health care personnel, and other essential workers as they experience frequent close contact with patients as well as the public.
"Authorized mRNA COVID-19 vaccines are effective for preventing SARS-CoV-2 infection in real-world conditions."
Findings confirm the benefits of current vaccination efforts in preventing SARS-CoV-2 infection
The findings also showed that partial immunization offered preventive benefits with 80% effectiveness, which agrees with Phase III trial results and multiple recent estimates of mRNA vaccine effectiveness for partial immunization with Pfizer-BioNTech vaccine in the U.K. and Israel. They are also consistent with early findings of SARS-CoV-2 employee and clinical testing by mRNA vaccination status in the U.S.
The significance of these findings is enhanced by the prospective design of the study and the high adherence of the participants to weekly specimen collection. These results on interim vaccine effectiveness for both Moderna's and Pfizer-BioNTech's mRNA vaccines in real-world conditions demonstrate that current mass vaccination efforts offer considerable benefits in preventing SARS-CoV-2 infection in working-age adults. The findings also reinforce the CDC's recommendation of a full 2-dose immunization with approved mRNA vaccines.
"COVID-19 vaccination is recommended for all eligible persons, which currently varies by location in the United States."
Thompson MG, Burgess JL, Naleway AL, et al. Interim Estimates of Vaccine Effectiveness of BNT162b2 and mRNA-1273 COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Health Care Personnel, First Responders, and Other Essential and Frontline Workers — Eight U.S. Locations, December 2020–March 2021. MMWR Morb Mortal Wkly Rep 2021;70:495–500. DOI: http://dx.doi.org/10.15585/mmwr.mm7013e3, https://www.cdc.gov/mmwr/volumes/70/wr/mm7013e3.htm?s_cid=mm7013e3_w