January 12, 2022

Vaccination offers long-lasting protections from the worst outcomes of COVID-19, according to a new study led by the UNC Gillings School of Global Public Health.

The emergence of new variants like delta and omicron has raised questions about whether breakthrough infections are caused by waning immunity from primary COVID-19 vaccination or by the more transmissible variants.

Results from the study published in the New England Journal of Medicine suggest that declining immunity over time is primarily responsible for breakthrough infections, but vaccination prevents hospitalization from severe illness and death nine months after getting the first shot.

Dr. Danyu Lin

Dr. Danyu Lin

“The primary takeaway message from our study is that unvaccinated people should get vaccinated right away,” said lead study author Danyu Lin, PhD, Dennis Gillings Distinguished Professor of biostatistics. “The results of our study also underscore the importance of booster shots, especially for older adults.”

The study, which is a collaboration between UNC-Chapel Hill and the North Carolina Department of Health and Human Services (NC DHHS), examined data on COVID-19 vaccination history and health outcomes for 10.6 million N.C. residents between December 2020 and September 2021.

This data included outcomes from COVID-19 cases caused by the delta variant. However, data from this study were collected before the discovery of the omicron variant.

“This is an excellent example of the wonderful research partnership between the Gillings School and NC DHHS, who are working together to generate the evidence base needed to keep our communities safe,” said Penny Gordon-Larsen, PhD, Carla Smith Chamblee Distinguished Professor of Global Nutrition and associate dean for research at the Gillings School.

The research was led by Lin with major contributions from Yu Gu, doctoral student in biostatistics, and Donglin Zeng, PhD, professor of biostatistics. NC DHHS epidemiologists Bradford Wheeler, MPH, Hayley Young, MPH, Shadia Khan Sunny, MD, PhD, MPH, and Zack Moore, MD, MPH, participated in the research. Shannon Holloway, PhD, from the NC State Department of Statistics also contributed.

“By applying a novel methodology to the rich surveillance data, we were able to provide precise and comprehensive characterization of the effectiveness over a nine-month period for the three vaccines employed in the U.S.,” Lin said.

“Unlike previous studies, we estimated the vaccine effectiveness in reducing the current risks of COVID-19, hospitalization and death as a function of time elapsed since the first dose,” Lin continued. “This information is critically important in determining the need for and the optimal timing of booster vaccination.”

The study found that effectiveness of the Pfizer and Moderna mRNA vaccines in reducing the risk of COVID-19 reached a peak of about 95 percent at two months after the first dose and then gradually declined.

At seven months, the Pfizer vaccine dropped to 67 percent effectiveness, compared to the Moderna vaccine, which maintained 80 percent effectiveness.

Among early recipients of the two mRNA vaccines, effectiveness dropped appreciably from mid-June to mid-July, when the delta variant was surging.

Effectiveness for the Johnson & Johnson adenovirus vaccine was 75 percent at one month after injection and fell to 60 percent after five months.

All three vaccines were durably effective at keeping vaccinated adults out of the hospital due to severe COVID-19. Effectiveness of the Pfizer vaccine reached a peak of 96 percent at two months and remained around 90 percent at seven months; effectiveness of the Moderna vaccine reached a peak of 97 percent at two months and remained at 94 percent at seven months. Effectiveness of the Johnson & Johnson vaccine reached a peak of 86 percent at two months and was higher than 80 percent through six months.

For the two mRNA vaccines, effectiveness against death was higher than that of hospitalization.

Earlier results from this study were cited by The Washington Post in an article on the importance of reaching the unvaccinated. The results were also used by the Centers for Disease Control and Prevention (CDC) to support the use of booster shots.

“Because the majority of the vaccines in the U.S. were administered more than seven months ago and only a small percentage of the population has received boosters, waning immunity is likely contributing to the breakthrough infections with the omicron variant,” Lin said.

“We are grateful for the partnership with UNC and for their extensive analysis of N.C. vaccination and case data,” said Moore, who is state epidemiologist and epidemiology section chief at NC DHHS. “This and other recent studies show that vaccination, including boosters, is our best defense against severe illness, hospitalization and death from COVID-19. This continues to be true as the virus evolves.”

Lin’s research team plans to analyze an updated version of the N.C. surveillance data soon. The updated data will provide valuable information about the effectiveness of the three COVID-19 vaccines over one year, the effectiveness of boosters and the impact of the omicron variant.

Everyone age five years and up are eligible for a COVID-19 vaccine. Those 18 and older should get a booster shot. Visit the NC DHHS website to find a vaccination clinic near you.

Contact the UNC Gillings School of Global Public Health communications team at sphcomm@unc.edu.

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