California has recorded about 1,400 breakthrough cases of COVID-19 in fully vaccinated people from Jan. 1 through April 21.

Breakthrough cases are defined as anyone who tested positive for COVID-19 more than 14 days after completing their vaccination series. According to the California Department of Public Health, breakthrough cases constitute about 0.1% of the approximately 1.4 million COVID-19 cases in that time frame.

“No vaccine is 100% efficacious,” said UC Berkeley infectious diseases professor emeritus John Swartzberg in an email. “These vaccines are nearly that. Still, for a small percentage of people, breakthrough cases may occur.”

There have been about 7,000 breakthrough infections nationwide out of more than 87 million fully vaccinated people, according to the Centers for Disease Control and Prevention. However, this number is an undercount because data on breakthrough cases relies on voluntary reporting from state health departments and may not be caught due to lack of testing, particularly for asymptomatic or mild cases.

While the number of breakthrough infections may sound like a lot, when compared to the entire number of those vaccinated, the number of breakthrough infections is “tiny,” according to Julia Schaletzky, executive director of the campus Henry Wheeler Center for Emerging and Neglected Diseases.

Breakthrough cases are most likely due to the vaccine not having elicited a sufficient immune response in the person, Schaletzky noted. This could result from improper injection of the vaccine or an insufficient response from the individual’s immune system.

Another potential cause of breakthrough cases is that the expansion of strains does not get eradicated by the vaccine as strongly as the original strain, though that has not been of significant concern yet, according to Schaletzky.

“Against all the variants of concern, the vaccine is effective,” Swartzberg said in the email.

Schaletzky added that the benefits of vaccines can be seen in the number of COVID-19 incidents and mortality rates of vaccinated people.

University Health Services, or UHS, is testing students living on campus once a week regardless of their vaccination status and is monitoring for breakthrough cases with contact tracing and genomic sequencing, according to Guy Nicolette, assistant vice chancellor of UHS.

Nicolette added that in the fall, UHS will continue to test unvaccinated individuals weekly and will randomly sample a small percentage of vaccinated individuals to monitor for breakthrough incidents.

Campus is factoring in several possibilities into its fall reopening plans, such as COVID-19 variants that evade some immunity, a lack of herd immunity or unexpected delays in the vaccine rollout, according to Nicolette.

While it is currently unknown how many students will arrive on campus without being fully vaccinated in the fall, UHS is preparing to administer many vaccinations in the early fall, Nicolete said in an email. He added that he hopes 95% or more of students will arrive already vaccinated.

Nicolette encouraged students to upload their vaccine information as soon as possible to their secure medical record, which is how UHS monitors vaccination status.