Explainer: COVID-19 vaccine shots add to confusion over China's tests for travellers

Although the British expert from the World Health Organization (WHO) subsequently tested negative, it was not immediately clear if the earlier result was a false positive, or the result of previous infection or a COVID-19 vaccination.

Here are details of China's testing rules, potential problems with the policy, its implications for vaccinated individuals and experts' views.

Travellers from many countries, such as Canada, Germany, Singapore and the United States, must show negative results of nucleic acid and Immunoglobulin M (IgM) antibody tests taken within 48 hours of boarding.

China uses IgM antibodies, normally detected early in infections, as a supplementary tool to filter out those who may have been infected but get a negative result on nucleic acid tests.

However, some cases showed IgM antibodies can persist for a longer time, and even after nearly complete recovery.

It is possible, but not always, experts say. Most vaccines target the "spike" protein on the virus surface to trigger an immune response that could include IgM antibodies.

"We can assume that any COVID-19 vaccine containing the spike protein will induce IgM and therefore a diagnostic assay designed to detect spike specific IgM will not be able to differentiate vaccination from infection," said Helen Fletcher, a professor of immunology at the London School of Hygiene & Tropical Medicine.

Published data on Oxford University/AstraZeneca Plc's COVID-19 vaccine shows spike protein-triggered IgM is detectable in some people at least 56 days after immunisation, Fletcher said.

Tests to detect antibodies triggered by non-spike protein can yield negative results for those who got vaccines targeting spike protein, said Jin Dong-Yan, a professor of virology at the University of Hong Kong.

Vaccines targeting spike protein include those from AstraZeneca, Pfizer Inc and its partner BioNTech, and Moderna.

Such tests, however, can be problematic for other types of vaccines, including whole virus-based shots that several Chinese developers use, some experts said.

"Where a person is injected with whole virus-based inactivated COVID-19 vaccine...there's a strong chance that the person may also have positive result from non-spike protein IgM antibody tests," said Ian Jones, a virologist at Britain's University of Reading.

China's health authority does not clarify whether its test is designed to detect antibodies triggered by spike protein or other protein. The National Health Commission did not respond to a request for comment.

China does not say clearly how it decides to exempt vaccinated people from its requirements for negative antibody tests.

Responding to a query on the topic, one of its Singapore embassy staff said a vaccinated foreigner can submit an immunisation certificate and await a response. China's Foreign Ministry declined to comment.

At November's G20 summit, Chinese President Xi Jinping proposed a global mechanism for mutual recognition of health certificates, including nucleic acid tests.

Experts hope China will consider using a vaccine passport to replace the antibody test.

"A vaccine passport will make travel to China much less of a hassle," said Huang Yanzhong, a health expert at U.S. think tank the Council on Foreign Relations.

"As Western countries achieve herd immunity through mass vaccination, they will start to open their borders... If China continues to have such strict test requirement, its airline, hotel and tourism industries will lose out."