DDC: Mixing vaccines only recommended in case of allergy

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Thai health authorities on Tuesday stuck to the line that mixing different Covid-19 vaccines for the two shots is only recommended if there is a severe reaction to the first one. The comments follow growing indications that swapping jabs can significantly boost patients’ immune response.

“If there is an extreme reaction from the first dose, whether suffocation or admission to hospital, then the healthcare worker is allowed to administer a different type of vaccine for the second dose,” a representative of the Department of Disease Control (DDC) told Thai Enquirer.

She declined to comment on whether mixing the vaccines as standard might help boost patients’ immunity and also address the logistical challenges facing the roll-out of the country’s inoculation programme, saying the allergy angle remains the DDC’s focus.

Researchers from Chulalongkorn have found that changing vaccine type for the second jab can produce an immune response in some cases, almost seven times greater than using the same one twice.

The positive findings corroborate research in several countries in past weeks. But the sample is small and research is ongoing, cautioned Dr Yong Poovorawan, the chief of the Centre of Excellence in Clinical Virology at Chulalongkorn University, in his Facebook post Sunday.

Five patients were tested in the Chula study, four with Sinovac and then AstraZeneca, and the fifth with the reverse order.

After a month, the patients’ immunity responses were compared with the average antibody response of others who have recovered from Covid, or who had received two full doses of Sinovac or one of AstraZeneca (no one in Thailand has yet received two doses of AstraZeneca).

The results were as follows:

  1. Two doses of Sinovac: 92.36 U/mL of antibodies
  2. One dose of Astrazeneca: 51.04 U/mL of antibodies
  3. Infected with Covid-19 and recovered: 60.86 U/mL of antibodies
  4. First dose Sinovac, second dose AstraZeneca: 615 U/mL of antibodies
  5. First dose AstraZeneca, second dose Sinovac: 221.1 U/mL of antibodies

Dr Yong said this could help where a patient is allergic to the brand of the first dose, but also more widely.

“In the case where there is a shortage of any of the vaccines, then we will be able to mix-and-match with the other brand of vaccine,” said Dr Yong. “The administration of vaccines would be much easier, faster, and efficient.”

The National Vaccine Institute also declined to comment on the potential of mixing vaccines to improve the logistics of the government’s programme.

In May, researchers from Spain found that mixing the AstraZeneca vaccine with Pfizer triggered a “potent immune response” from a trial of over 600 people.

Combining different vaccines is already common practice when inoculating children, explained Dr Yong. 

“Theoretically, vaccines are interchangeable when they are used on children, and all of this has been studied and proven,” including for rotavirus, Hepatitis A, Hepatitis B, diphtheria, and tetanus, he said.

“Germs cannot tell the difference between each brand of vaccination.”


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