LONDON • When Turkey was taken off Britain's red list for travel last month, Ms Sally Morrow, an English expatriate living in the Turkish capital of Ankara, rushed to her computer and booked flights to London, so that she could reunite with her ailing parents after more than six months apart.

But when her ticket confirmation came through, Ms Morrow, 47, read that the certificate she received when she was vaccinated in Turkey - with the Pfizer coronavirus vaccine - would not be accepted in Britain. As a result, she would have to quarantine for 10 days and have at least three negative coronavirus tests before being permitted to leave isolation there.

"I had the Pfizer jab, the Rolls-Royce of vaccines, the exact same one as millions of Brits, yet I'm considered unvaccinated simply because I got my vaccine abroad," Ms Morrow said.

"It's outright discrimination, and it's a disgrace. What do they think? That Turks are selling knock-off vaccines at the Grand Bazaar?" she said, referring to the Ottoman-era market in Istanbul known for selling counterfeit designer merchandise.

Over the summer, many countries opened to international visitors following the successful roll-out of vaccination programmes, but fragmented rules about which vaccines will be accepted and what documentation is required, as well as a lack of compatibility between vaccine apps, have left many travellers confused and frustrated over where they can visit without extraordinary headaches and restrictions.

More than 2.7 billion people around the world have been fully vaccinated with a range of vaccines that vary in degrees of efficacy, according to Our World in Data, an Oxford University Covid-19 database.

Across Asia, the United Arab Emirates and South America, millions have received Sinopharm, Sinovac and other China-made vaccines, but concern over their efficacy has resulted in many countries not recognising them for the purpose of travel. Millions more who received domestic vaccines such as Sputnik V in Russia and Covaxin in India, which have not received approval from the World Health Organisation (WHO), are also limited in where they can go.

Britain eased its travel rules last week, expanding the list of vaccination certificates it recognises from other territories, such as Turkey and India, but certificates from many nations in Africa and South America were excluded.

In terms of vaccines, Britain, the 27-member European Union and the 26-country Schengen Area accept the four vaccines approved by the European Medicines Agency - AstraZeneca, Pfizer, Moderna and Johnson & Johnson - but the UK and many EU states do not recognise the Sinopharm and Sinovac vaccines, despite their approval by the WHO.

The United States is still in a "regulatory process" to determine which vaccines it will accept when the country opens to fully vaccinated travellers in November, the Centres for Disease Control and Prevention said in a statement.

But vaccines listed for emergency use by the WHO, including AstraZeneca, will be recognised, the agency said. These confusing rules over approved vaccines are not limited to Britain and the US.

Experts warn that the haphazard and preferential approach to travel regulations is creating a two-tier system in which people vaccinated with the most effective vaccines - mainly in the West - are able to cross borders freely, while those in developing countries who have received vaccines with a lower efficacy, are not.

They fear that such policies will contribute to immunisation hesitancy in parts of the world where the most widely accepted vaccines are not available.

Ms Anita Engel, 45, a German national who works in Dubai, received her second dose of the Sinopharm vaccine in the UAE in June. She then got two shots of the Moderna vaccine at home in Germany in August, and experienced severe side effects after her second dose of the Moderna vaccine.

A doctor told her that she was having an adverse reaction to the high level of antibodies in her body, caused by mixing different vaccines in a short amount of time, and that she should have gotten an antibody test before getting a third dose. "I felt stupid for taking a risk, but I won't lie - it feels really good to be able to travel again," said Ms Engel.

NYTIMES