As of August 6, 2021, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is the virus responsible for the coronavirus disease 2019 (COVID-19), has infected over 201 million people and caused the deaths of almost 4.3 million worldwide.

The high transmissibility and unpredictable nature of SARS-CoV-2 has supported the rapid development of several COVID-19 vaccines that have already been distributed in many countries around the world. To date, almost 30% of the world’s population has received at least one dose of a COVID-19 vaccine.

The recent rise in COVID-19 cases

The rise in SARS-CoV-2 cases in Israel, as well as many other countries around the world, requires a better understanding of how SARS-CoV-2 immune protection can weaken over time in both vaccination individuals and those who are convalesced seropositive as a result of previous SARS-CoV-2 infection. To this end, a recent study published on the preprint server medRxiv* determines whether the time elapsed since the second dose of the BNT162b2 vaccine is associated with an increased risk of breakthrough SARS-CoV-2 infection.

About the study

The current study is a retrospective cohort study that is based on data collected from the Leumit Health Services (LHS), which is a large healthcare provider in Israel. The cohort included 33,993 individuals who had received a reverse transcriptase polymerase chain reaction (RT-PCR) test between May 15, 2021, and July 26, 2021, at least two weeks after receiving their second dose of the COVID-19 vaccine.

To reflect the vaccine rollout stages, the researchers included three age groups in the study, which included individuals who were 60 or older, between the ages of 40 and 59, and between the ages of 18 and 39. Importantly, at the time of their RT-PCR test, none of the study participants had any evidence of previous COVID-19 infection.

Study results

The median time between when the participants had received the second dose of the COVID-19 vaccine and received an RT-PCR test was 146 days. However, over 50% of the participants had received an RT-PCR test over 146 days since their second dose.

Using 146 days as their cut-off, the researchers found that within the 60 and over age group, 2.19% of vaccinated individuals tested positive for SARS-CoV-2. Comparably, the positivity rate in the 40-59 and 18-39 age groups was 1.93% and 1.39%, respectively. Taken together, the increase in SARS-CoV-2 infection rates across all patient populations was significant.

Notably, at the time when the study was conducted, the dominant circulating variant was the SARS-CoV-2 Delta variant B.1.617.2. Of the 113 isolates that were sent for sequencing in this study, 93% tested positive for the Delta variant.