In the face of new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants such as Delta, reasonable concerns have been raised over how COVID-19 vaccines are affected. But new research finds people continue to have high vaccine effectiveness until around 6 months after being vaccinated.

More than 4.9 million individuals were assessed for eligibility between Dec 14, 2020, and Aug 8, 2021, and over 3.4 million were included in the study.

SARS-CoV-2 infection protection of fully vaccinated individuals was found to be 73%, and COVID-19-induced hospitalization protection was 90%. The effectiveness against infections dropped from 88% during the first month following full vaccination to 47% after 5 months.

The vaccine's effectiveness against the delta variant sequenced infections was high in the first month following full vaccination (93%) but declined after four months to 53%. Vaccination was also 97% effective against other variants (non-delta) the first month after the full vaccination but dwindled to 67% at the end of 4–5 months. Vaccination against delta variant infections has been effective in reducing hospital admissions by 93 percent nationwide up to 6 months.

"Our results provide support for high effectiveness of BNT162b2 against hospital admissions up until around 6 months after being fully vaccinated, even in the face of widespread dissemination of the delta variant," concluded the researchers.

Reduced vaccine effectiveness against COVID-19 infection may likely be due to waning immunity rather than the Delta variant. However, given the decline in immunity, the researchers suggest booster doses are potentially needed to reinstate high protection levels.

The study "Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study" is published in the journal The Lancet.

Study: Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study. Image Credit: Rido / Shutterstock

How they did it

The researchers monitored patients in the Kaiser Permanente Southern California system. All patients eligible for the study were over 12 years old. The median age was 45, about 52.4% of participants were female, and about 40.5% were Hispanic.

They compared the risk of COVID-19 infection — confirmed with a PCR test — between unvaccinated and vaccinated individuals.

In individuals with COVID-19 infection, samples were taken and sequenced to evaluate for SARS-CoV-2 variants. The prevalence of specific variants differed by season. The Epsilon variant dominated a majority of infections in the winter, Alpha took over in the spring, and Delta came on to the scene in the summer.

Waning immunity against COVID-19 infection over time

Vaccines remain effective but not as strong as the time of vaccination. Throughout the study, about 5.4% of participants were infected with SARS-CoV-2. Of this, 6.6% required hospitalization. These individuals were more likely to be older, male, and with a history of comorbidities.

Results showed that vaccine effectiveness in preventing COVID-19 infection was 91% for ages 12-15 years old and 61% for people 65 years and older. Vaccine effectiveness against hospitalization after full vaccination was 92% for people 16 to 44 years old. In people 65 years and older, the vaccine was 86% effective for people over 65.

After 1 month of vaccination, vaccine effectiveness against infection lowered to 88%. After 5 months, vaccine effectiveness against infection decreased to 47%.

For people over 65 years, vaccine effectiveness against infection dropped from 80% after 1 month to 43% after 5 months.

Regardless of the SARS-CoV-2 variant, a waning effect continued to be seen over time. For example, while the Pfizer-BioNTech vaccine showed high protection against SARS-CoV-2 variants after a month, vaccine effectiveness against Delta dropped to 53%. For other variants, vaccine effectiveness against other variants decreased to 67%.

Pfizer-BioNTech vaccine continued to show robust protection against severe illness

However, effectiveness against hospitalization remained steady. In preventing hospital admission for COVID-19, vaccine effectiveness showed no significant waning with the effectiveness of 87% after 1 month and 88% after 5 months.

"Effectiveness against hospital admissions in all age groups did not wane over the duration of the study. These findings are consistent with preliminary reports from the Israel Ministry of Health and US Centers for Disease Control and Prevention showing reductions in the effectiveness of BNT162b2 against infections 5 months or longer after being fully vaccinated, but consistently high estimates against COVID-19- related hospital admissions and severe disease up until July, 2021," wrote the research team.

Written by

Jocelyn Solis-Moreira

Jocelyn Solis-Moreira graduated with a Bachelor's in Integrative Neuroscience, where she then pursued graduate research looking at the long-term effects of adolescent binge drinking on the brain's neurochemistry in adulthood.