Should young people who’ve had one AZ shot switch to Pfizer for second?

Young South Koreans who have received their initial shots of AstraZeneca’s COVID-19 vaccine are calling for access to the Pfizer vaccine for their second shots, now that people under 30 are no longer eligible for the AstraZeneca vaccine over rare blood clot risks.

Of the around 135,000 people in their 20s who have received the AstraZeneca vaccine, most are health care workers. As such, they were at the top of the vaccine priority list when the immunization campaign kicked off in late February. 

Now they are wondering if they should get a second dose when use of the vaccine has been discontinued for people in their age group -- not just in Korea, but also in several other countries including France, Germany and the UK. 

Korea’s Ministry of Health and Welfare still maintains that people younger than 30 who received a first AstraZeneca shot complete their immunization with a second of the same vaccine.

A nurse at a top-ranking hospital in Seoul, who got his first AstraZeneca shot in March, said over the phone that he was “unsure” about getting a second. “If this vaccine is being phased out, or scrapped from use in other countries for people my age, then I don’t know if I should receive it.”

A thread posted Monday in a private Facebook group for nurses conveyed similar concerns. “They’re now accepting appointments for the second shot. I’m already nervous,” wrote one member who said she was in her mid-20s.

In an April 8 briefing, held a day after the Korea Disease Control and Prevention Agency temporarily paused the AstraZeneca vaccine’s use in people under 60, the agency said in response to a reporter’s inquiry that second doses with the Pfizer product “could be a possibility.”

Yet on Wednesday, Ministry of Health and Welfare spokesperson Son Young-rae said the approach of mixing and matching different vaccines was being “carefully considered” but that it was “not scientifically supported based on a general understanding of how vaccines work.”

“Since COVID-19 vaccines are new, and this is an unprecedented situation, new strategies are being tried and new facts and data are emerging every day,” he said.

But there is a precedent for mixing different types of vaccines, according to infectious disease professor Dr. Kim Woo-joo of Korea University. “These approaches date back decades,” he said. They have been tested in early HIV vaccine development efforts, and there was one Ebola vaccine that used a mix of different platforms.

The mixture of doses could also help a person avoid becoming immune to the vaccine itself, he explained. As AstraZeneca’s COVID-19 shot uses an adenovirus, it’s possible to develop immunity against it after a second exposure -- in which case the vaccine might fail to work should a third shot is needed to defend against variants.

Virologist Dr. Paik Soon-young of Catholic University of Korea said, “Since people under 30 are currently excluded from AstraZeneca vaccinations, administering them with the second dose of the vaccine would go against the guidelines.”

Dr. Jean-Louis Excler, an expert at the COVAX vaccine safety working group, said mixing vaccines within a two-dose regimen “seems to be a bit more reactogenic (incurring side effects), but worth it because it’s likely to be more efficacious (eliciting a stronger or longer-lasting immune response).”

By Kim Arin (arin@heraldcorp.com)