An admin error led to people in south Sefton being given the "wrong" type of covid vaccine for their second dose.
Current guidance says that people in the UK must receive the same type of vaccine for both doses, while studies are undertaken into the effects of "mixing and matching" doses.
However South Sefton NHS Clinical Commissioning Group (CCG) is investigating after the admin cock-up at an unnamed vaccination site in the area led to the wrong vaccine types being recorded for 41 patients.
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The error led to three people receiving "mixed vaccine schedules", which was recorded as a serious untoward incident by NHS managers.
So far, the Medicines and Healthcare products Regulatory Agency (MHRA) has approved vaccines developed by Pfizer/BioNtech, Oxford/AstraZeneca and Moderna, which all require a second dose several weeks after the first.
The regulator last month also approved a single dose jab developed by Johnson & Johnson, but this is not yet widely available.
A report from South Sefton interim chief nurse Chrissie Cooke to the CCG board said the admin error was spotted on May 19 and alerts were submitted to NHS England and the System Vaccine Operations Centre (SVOC).
She wrote: "This affected 41 individuals and the accurate recording of the vaccine received at their first dose.
"Consequently three people have received a mixed vaccine schedule, which is outside of local COVID vaccine administration arrangements."
Ms Cooke said an investigation is ongoing and the affected patients have been made aware.
It was reported in April that mixing vaccine doses may actually offer an increased immune response, offering more protection from coronavirus, although scientists said more research was needed.
A spokeswoman for South Sefton CCG, said: "Due to an administration error that took place in February, three people have received a mixed vaccine schedule.
"All individuals affected have been informed and steps have been taken to prevent this from happening again.
"Thousands of people in Sefton have already received their lifesaving vaccine.
"If you’re aged 18 or over you can book an appointment now, or find a walk-in vaccination site near you, through the national booking service.
"If you have had a first dose, make sure to book your second dose to ensure that you have maximum protection from the virus."
