NEW YORK (NYTIMES) - As poor countries around the world struggle to beat back the coronavirus, they are unintentionally contributing to fresh explosions of illness and death from other diseases - ones that are readily prevented by vaccines.

This spring, after the World Health Organisation (WHO) and Unicef warned that the pandemic could spread swiftly when children gathered for shots, many countries suspended their inoculation programmes. Even in countries that tried to keep them going, cargo flights with vaccine supplies were halted by the pandemic and health workers diverted to fight it.

Now, diphtheria is appearing in Pakistan, Bangladesh and Nepal. Cholera is in South Sudan, Cameroon, Mozambique, Yemen and Bangladesh.

A mutated strain of poliovirus has been reported in more than 30 countries. And measles is flaring around the globe, including in Bangladesh, Brazil, Cambodia, Central African Republic, Iraq, Kazakhstan, Nepal, Nigeria and Uzbekistan.

Of 29 countries that have suspended measles campaigns because of the pandemic, 18 are reporting outbreaks. An additional 13 countries are considering postponement. According to the Measles and Rubella Initiative, 178 million people are at risk of missing measles shots in 2020.

The risk now is "an epidemic in a few months' time that will kill more children than Covid," said Dr Chibuzo Okonta, president of Doctors Without Borders in West and Central Africa.

As the pandemic lingers, the WHO and other international public health groups are now urging countries to carefully resume vaccination while contending with the coronavirus.

At stake is the future of a hard-fought, 20-year collaboration that has prevented 35 million deaths in 98 countries from vaccine-preventable diseases, and reduced mortality from them in children by 44 per cent, according to a 2019 study by the Vaccine Impact Modeling Consortium, a group of public health scholars.

"Immunisation is one of the most powerful and fundamental disease prevention tools in the history of public health," said Dr Tedros Adhanom Ghebreyesus, director-general of the WHO, in a statement. "Disruption to immunisation programmes from the Covid-19 pandemic threatens to unwind decades of progress against vaccine-preventable diseases like measles."

But the obstacles to restarting are considerable. Vaccine supplies are hard to come by. Health care workers are increasingly working full time on Covid-19, the infection caused by the coronavirus. And a new wave of vaccine hesitancy is keeping parents from clinics.

Many countries have yet to be hit with the full force of the pandemic itself, which will further weaken their capabilities to handle outbreaks of other diseases.

"We will have countries trying to recover from Covid and then facing measles. It would stretch their health systems further and have serious economic and humanitarian consequences," said Dr Robin Nandy, chief of immunisation for Unicef, which supplies vaccines to 100 countries, reaching 45 per cent of children under 5.

The breakdown of vaccine delivery also has stark implications for protecting against the coronavirus itself.

At a global summit earlier this month, Gavi, the Vaccine Alliance, a health partnership founded by the Bill and Melinda Gates Foundation, announced it had received pledges of US$8.8 billion (S$12.26 billion) for basic vaccines to children in poor and middle-income countries, and was beginning a drive to deliver Covid-19 vaccines, once they are available.

But as services collapse under the pandemic, "they are the same ones that will be needed to send out a Covid vaccine," warned Dr Katherine O'Brien, the WHO's director of immunisation, vaccines and biologicals, during a recent webinar on immunisation challenges.

This had been the year that Congo, the second-largest country in Africa, was to launch a national immunisation programme.

The urgency could not have been greater. The measles epidemic in the country, which started in 2018, has run on and on: Since January, there have been more than 60,000 cases and 800 deaths. Now, Ebola has again flared, in addition to tuberculosis and cholera, which regularly strike the country.

Vaccines exist for all these diseases, although they are not always available. In late 2018, the country began an immunisation initiative in nine provinces. It was a feat of coordination and initiative, and in 2019, the first full year, the percentage of fully immunised children jumped from 42 per cent to 62 per cent in Kinshasa, the capital.

This spring, as the programme was being readied for its nationwide rollout, the coronavirus struck. Mass vaccination campaigns, which often mean summoning hundreds of children to sit close together in schoolyards and markets, seemed guaranteed to spread coronavirus. Even routine immunisation, which typically occurs in clinics, became untenable in many areas.

The country's health authorities decided to allow vaccinations to continue in areas with measles but no coronavirus cases. But the pandemic froze international flights that would bring medical supplies, and several provinces began running out of vaccines for polio, measles and tuberculosis.

When immunisation supplies finally arrived in Kinshasa, they could not be moved around the country. Domestic flights had been suspended. Ground transport was not viable because of shoddy roads. Eventually, a UN peacekeeping mission ferried supplies on its planes.

Measles virus spreads easily by aerosol - tiny particles or droplets suspended in the air - and is far more contagious than the coronavirus, according to experts at the Centres for Disease Control and Prevention.

"If people walk into a room where a person with measles had been two hours ago and no one has been immunised, 100 per cent of those people will get infected," said Dr Yvonne Maldonado, a pediatric infectious disease expert at Stanford University.

In poorer countries, the measles mortality rate for children under 5 ranges between 3 per cent and 6 per cent; conditions like malnutrition or an overcrowded refugee camp can increase the fatality rate. Children may succumb to complications such as pneumonia, encephalitis and severe diarrhea.

In 2018, the most recent year for which data worldwide has been compiled, there were nearly 10 million estimated cases of measles and 142,300 related deaths. And global immunisation programmes were more robust then.

Before the coronavirus pandemic in Ethiopia, 91 per cent of children in the capital of Addis Ababa received their first measles vaccination during routine visits, while 29 per cent in rural regions got them. (To prevent an outbreak of a highly infectious disease like measles, the optimum coverage is 95 per cent or higher, with two doses of vaccine.) When the pandemic struck, the country suspended its April measles campaign. But the government continues to report many new cases.

"Outbreak pathogens don't recognise borders," said Dr O'Brien of the WHO. "Especially measles: Measles anywhere is measles everywhere."