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Some Covid-19 vaccines give recipients antibodies to help their body fight the coronavirus. Photo: AP

Covid-19 vaccines may give you antibodies that prevent you entering China

  • China requires inbound air passengers to test negative for a viral antibody as well as for Covid-19 – but some have acquired that antibody from a vaccine
  • Travellers, who must get clearance from Chinese embassies before boarding flights, say they face uncertainty
When 26-year-old Peter Zhou graduated from medical school in New York and wished to return to Shanghai last month, he was faced with a dilemma: the antibody protection he received from a Covid-19 vaccine jab might be the very reason he could not go home.

In November, China started to insist inbound passengers produce evidence of testing negative for the antibody immunoglobulin M (IgM) – usually the first type of antibody produced against virus infection – along with a negative Covid-19 test taken within 48 hours before boarding.

Chinese embassies in the departure country assess this documentation and, if satisfied, issue a green health code via a mobile app to clear the way to board a flight.

Zhou took the Pfizer-BioNTech vaccine early last month and planned to fly to China from Dallas two weeks later. He filed his positive IgM antibody test result to a Chinese embassy in the US together with a document proving he had taken his first vaccine dose, hoping it would be taken into consideration. He did not receive the green code.

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“I had to return to my … flat. I spent about US$800 on testing, hotel accommodation and air tickets in vain,” Zhou said.

As Covid-19 vaccines are rolled out globally, people with plans to travel to China are struggling with the same dilemma like Zhou – whether the antibodies developed after taking a jab will impede their trip because of the country’s strict travel restrictions.

“People do make both IgM and IgG after both infections and [taking the] vaccines, and both groups are expected to test positive,” said Deepta Bhattacharya, associate professor of immunobiology with University of Arizona College of Medicine, referring to two different types of antibodies created by the human body’s immune system to fight off infection with an invading pathogen such as the coronavirus that causes Covid-19.

The level of IgM wanes in time and is later replaced by other types of antibodies such as immunoglobulin G (IgG). The US Centres for Disease Control and Prevention does not recommend using an antibody test as a diagnosis for current infection because it can take one to three weeks after the infection for the human body to produce antibodies.

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There was still no long-term data to indicate how long IgM was present after vaccination, but IgG could be around for many months after vaccination and was more likely to be for years, Bhattacharya said.

In a statement to the South China Morning Post, China’s foreign ministry said: “The issue of producing positive antibody results after vaccination is complicated. China has made relative arrangements based on opinions of experts to ensure people that have taken vaccines can enter China smoothly when satisfying relative requirements.

“The relevant epidemic control measures will be adjusted as the global epidemic situation changes.”

Some individual embassies said inbound passengers needed to provide documentation proving they took Covid-19 vaccines for the authority to consider whether to issue a green health code, but others were silent on the issue.

“Chinese or foreign nationals who have taken Covid-19 vaccines can provide a proof document of vaccination on top of [Covid-19] test results to be reviewed for health code,” said a statement from Chinese embassy in Kyrgyzstan.

Similar statements were given by Chinese embassies in Singapore, New Zealand, Hungary and Czech Republic that travellers to China can upload their vaccination document together with antibody result, if it is positive, to apply for a health code.

The social media app to apply for the health code added questions on whether the applicant had taken a Covid-19 vaccine and also listed the vaccines available in roll-outs globally.

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The embassy in Britain, however, specified that only people with both negative antibody and nucleic results, regardless if they have taken Covid-19 vaccines, can apply for the health code.

On social media, anxious potential travellers who have been vaccinated overseas discussed whether to wait for the level of IgM to become undetectable or find ways to prove the antibody was produced from vaccination instead of coronavirus infection.

Alice Ma, 25, who had two Moderna shots last month after working in a Covid-19 testing clinic in New York, hoped her antibody IgM would be undetectable before her flight to China in April. She has bought another ticket for May as a precaution.

“I feel anxious as the flight draws near, because I need to make detailed plans for going back to China, with many moving parts – but the process is beyond my control,” she said. “I hope an official policy on vaccinated people can be announced soon to save us the trouble.”

Bhattacharya said that there are ways to differentiate the positive IgM between a recent vaccination or virus infection, but it only applies to the vaccines that targets solely the spike protein of the coronavirus.

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“For Pfizer-BioNTech, Moderna, Johnson & Johnson, Novavax and AstraZeneca, the vaccines only encode the spike protein, not other viral proteins like nucleocapsid. So infections would induce antibodies against nucleocapsid, but the vaccines would not,” said Bhattacharya. “Tests that measure nucleocapsid antibodies, therefore, could theoretically distinguish infection from vaccination-induced immunity.”

For other vaccines such as CoronaVac by Sinovac, which contain the whole virus that has been inactivated, there is no test that could distinguish between infection- and vaccine-induced antibodies, according to Bhattacharya.

Zhou took the time to find a clinic that tests for antibodies against the nucleocapsid protein instead of the spike protein by looking at what test kits were being used by embassy recommended clinics and finding product details on the US Food and Drug Administration website.

With a negative antibody test against the nucleocapsid protein, Zhou boarded a plane to Shanghai earlier this month and began his 14 days of quarantine.

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Jessica Jiao, who worked in a hospital in New York and was inoculated last month with the Moderna vaccine, got the green health code even with a positive IgM test result. She attributed that to her efforts to prove the antibody was from vaccination, not infection.

She submitted hospital medical records showing she was there for inoculation, and a hospital telephone number so the embassy could check.

“I know some complained they did not get the health code and I understand the need for restrictive requirements,” Jiao said from her hotel quarantine. “I feel both sides have a good argument, so I can only do my best to prove the antibody was from a vaccine.”

Bhattacharya said antibody tests for detecting a recent infection make less sense after the roll-out of vaccines because antibodies arise a few weeks into the infection at a point where people tend to be much less contagious.

He said a better option for a second test besides the PCR (polymerase chain reaction) tests would be for viral antigens, which return results quickly and, along with PCR, are more direct measures of whether someone still has the infectious virus or is shedding the virus.

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