Designated as a priority group, healthcare workers were among the first to be vaccinated when inoculations began in January.
Almost all have received the COVID-19 vaccine developed by Chinese biopharmaceutical company Sinovac, the Indonesian Medical Association (IDI) says.
While the number of Indonesian healthcare workers dying from COVID-19 has dropped sharply from 158 in January to 13 in May, according to data initiative group LaporCOVID-19, public health experts say the Java hospitalisations are cause for concern.
"The data shows they have the Delta variant (in Kudus) so it is no surprise that the breakthrough infection is higher than before, because, as we know, the majority of healthcare workers in Indonesia got Sinovac, and we still don’t know yet how effective it is in the real world against the Delta variant,” said Dicky Budiman, an epidemiologist at Australia's Griffith University.
Spokespersons from Sinovac and Indonesia’s ministry of health were not immediately available for comment on the efficacy of the Chinese firm's CoronaVac against newer variants of the virus.
The World Health Organization (WHO) approved emergency use of Sinovac's vaccine this month, saying results showed it prevented symptomatic disease in 51% of recipients and prevented severe COVID-19 and hospital stays in all of those studied.
As Indonesia grappled with one of Asia's worst outbreaks, registering more than 1.9 million infections and 53,000 deaths, its doctors and nurses suffered a heavy toll of 946 deaths.
Many are now experiencing pandemic fatigue and taking a less vigilant approach to health protocols after being vaccinated, said Lenny Ekawati, from LaporCOVID-19.
"That phenomenon happens quite often these days, not only within the community, but also healthcare workers," she said. "They think because they are vaccinated that they are safe."
But as more cases of the highly transmissible Delta variant are identified in the world’s fourth most populous nation, the data is starting to tell a different story.
Across Indonesia, at least five doctors and one nurse have died from COVID-19 despite being vaccinated, according to the data initiative group, although one had only received a first shot.
In Kudus, one senior doctor has died, said IDI, although it is understood he had a co-morbidity.
In Jakarta, the capital, radiologist Dr Prijo Sidipratomo told Reuters he knew of at least half a dozen doctors who had been hospitalised with COVID-19 in the past month, despite being vaccinated, with one now being treated in an ICU.
"It is alarming for us because we cannot rely on vaccinations only," he said, urging people to take precautions.
Weeks after the Muslim Eid Al-Fitr holidays, Indonesia has experienced a surge in cases, with the positivity rate exceeding 23% on Wednesday and daily cases nearing 10,000, its highest since late February.
In its latest report, the WHO urged Indonesia to tighten its lockdown as increased transmission due to variants of concern and a surge in bed occupancy rates demanded urgent action.
(Reporting by Kate Lamb in Sydney and Agustinus Beo Da Costa and Stanley Widianto in Jakarta; Editing by Michael Perry and Clarence Fernandez) ((Kate.Lamb@thomsonreuters.com;))