LONDON (NYTIMES) - 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 soon after 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, Ms Morrow would be required 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.
Lower efficacy, more stringent 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 vaccines manufactured in China, 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 this week, expanding the list of vaccination certificates it recognises from other countries and territories, including Turkey and India, but certificates from many nations in Africa and South America were excluded. In terms of vaccines, the United Kingdom, 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 Britain 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. The Sputnik V vaccine, which has been approved in more than 70 countries but not yet by the WHO, is unlikely to be accepted by the US as it initially reopens for international travel.
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.
Dr Melinda Mills, director of the Leverhulme Centre for Demographic Science at the University of Oxford and a lead author of a Royal Society report reviewing the feasibility of vaccine certificates, called the rules being developed "opaque and contradictory" and said they were "leaving people very frustrated".
"We are seeing cracks in these regulations where a country is on the red list of one country and on the green list of another, or when one type of vaccine is accepted by some countries but not by others," she said.
"And many of these systems are not designed to handle people of multiple nationalities and those who work across borders."
'Most places treat you like you are unvaccinated'
To circumvent restrictions, some multinational travellers have received additional doses of different vaccines in another country - vaccines that are more widely accepted around the world.
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 when she went home to Germany in August.
"The world opened up this summer, but I couldn't go anywhere with the Sinopharm without having to quarantine or take PCR tests. Most places treat you like you are unvaccinated," Ms Engel said.
"I got the Moderna in Germany, so that I could travel around Europe and reconnect with my friends, but I also feel safer because it provides more protection against the variants."
Ms Engel 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.
"He told me I should have taken an antibody test before I had my third vaccine, and that I shouldn't have made the decision without consulting a medical professional," she said.
"I felt stupid for taking a risk, but I won't lie - it feels damn good to be able to travel again."
The data on the efficacy and safety of mixing and matching vaccines is limited. An expert panel with the WHO has issued a recommendation to use the Pfizer or Moderna vaccine as a second dose following a first dose of the AstraZeneca vaccine, if a second dose of the same vaccine is not available.
A clinical trial led by the University of Oxford has shown that mixing the AstraZeneca and Pfizer vaccine generates a robust immune response against the coronavirus.
In the initial stages of vaccination rollouts in the spring, some countries offered mixed doses of vaccines because of supply shortages. In Canada, at least 3.9 million people received two different shots and are now facing travel restrictions because many countries, including the US, only consider people with two identical doses fully vaccinated.
"For Canadians, I think a booster shot to match one of the first vaccines doses will be the likely get-out-of-jail card," said Dr Mills, who is Dutch Canadian.
And where is your documentation?
Even after being permitted to enter a country, foreign visitors can face difficulties accessing establishments or services that require vaccine "passports" or certificates, such as restaurants and museums, because of compatibility issues between types of verification software.
On vacation in August, Mr Jason Trenton, a 49-year-old music technician who got vaccinated in New York in April, was turned away from a restaurant in Paris because the host was unable to scan his Excelsior Pass, a mobile app issued by New York state.
In June, France launched its own app, called the Health Pass, which stores vaccine certificates and PCR test results and is required to access bars, restaurants, shopping centers, tourist sites and public transport. At the time of Mr Trenton's visit, the system was not available to US citizens.
"Most places accept the CDC paper vaccine card, but I didn't want to carry it around with me because it's so easy to lose," Mr Trenton said.
"It's all down to chance, and you just have to hope that someone will accept your pass without scanning it. It worked in most places, but it's stressful because you make reservations and plan your day but you don't know if it will work out."
In some countries such as Switzerland, travellers who are not from the surrounding EU must apply for domestic vaccine certificates that are needed for indoor dining and cultural activities, but getting one can take up to seven days.
"It's all very unnecessary and confusing," said Mr John Morris, 59, an English teacher who lives in Istanbul. He has decided not to go home until Britain recognises the Pfizer vaccine he received in Turkey.
"These rules are just prejudiced against developing countries. I got this very good vaccine under a very efficient health system in Turkey, and I will travel to wherever accepts it."
Ms Morrow, however, grew fed up of waiting to see her parents and boarded her flight to London this week. She is quarantining at a friend's house for five days before she can take a PCR test that will allow her to leave isolation early.
"I'm sharing a house with my friend Wendy, who has exactly the same vaccine as me, but yet for some reason that makes absolutely no sense, if she goes abroad and comes back she's free to go straight to the pub, while I have lock myself up for days," Morrow said.
"These governments need to make rules that actually make sense and are backed up by data if they want people to respect and follow them."