As the coronavirus pandemic reared its ugly head around the world, health systems took a hit in more ways than one.

For instance, millions of children missed at least one vaccination in 2021 alone.

It led to "prevalent and severe" outbreaks of preventable diseases such as measles, diphtheria, polio and yellow fever, according to UN's health agency WHO.

The slowing of immunization during the pandemic left over 25 million children in especially lower-income countries vulnerable.

Now, with a renewed sense of urgency, global and national health partners have joined forces to urge worst-hit nations to play catch-up.

On Monday, partners announced The Big Catch-up, a new global effort aimed at boosting vaccination among children following declines driven by the coronavirus pandemic.

WHO and UN's children's agency UNICEF are involved.

Also part of this effort are GAVI, the Vaccine Alliance, as well as the Bill & Melinda Gates Foundation.

Community health workers walk door to door during a polio immunization campaign in Kiamako, Nairobi, Kenya on July 19, 2021

A Monday joint press statement said The Big Catch-up intends to place the most emphasis on "the 20 countries where three quarters of the children who missed vaccinations in 2021 live".

Eastern African nations Tanzania, DR Congo, Ethiopia and Somalia are on this list.

The others are Nigeria, Cameroon, Chad, Mozambique, Madagascar, Angola, India, Pakistan, Afghanistan, Indonesia, Philippines, Myanmar, Vietnam, North Korea, Brazil and Mexico.

A community health worker administers an oral polio vaccine during a door-to-door polio immunization campaign in Mbezi Makabe, Dar es Salaam on May 21, 2022

Despite the drop in global immunization coverage levels, "bright spots of resilience" were recorded in some countries.

Uganda, for instance, is said to have maintained high coverage levels during the pandemic.

Countries like Kenya are reported to have done well to reach groups in vulnerable situations to improve levels of immunization through "collaborations with community health workers and local leaders".

'Public health triumph'

According to WHO director-general Dr Tedros Adhanom Ghebreyesus, no child should die of a disease that is vaccine-preventable.

Dr Ghebreyesus says WHO is supporting countries to restore immunization

"Catching up is a top priority," said Ghebreyesus.

"WHO is supporting dozens of countries to restore immunization and other essential health services".

UNICEF executive director Catherine Russell said children who do not get their early vaccines are "at added risk of being cut out of health care in the long run".

“The longer we wait to reach and vaccinate these children, the more vulnerable they become and the greater the risk of more deadly disease outbreaks. Countries, global partners and local communities must come together to strengthen services, build trust and save lives," she said.

Dr Seth Berkley, the CEO of GAVI, made a similar call, saying all stakeholders must work to "protect the life of every child".

“We cannot allow a legacy of the pandemic to be the undoing of many years’ work protecting more and more children from deadly, preventable diseases," he said.

Dr Seth Berkley, the CEO of GAVI, The Global Alliance

Also commenting on the new joint effort, Dr. Chris Elias, president of global development at the Bill & Melinda Gates Foundation, called vaccines a "public health triumph".

“The incredible progress that has been made toward ending polio and reducing the incidence of infectious diseases is the direct result of thousands of dedicated global partners and local health workers who have worked to immunize millions of children," he said.

"We must double down to reach all children with the vaccines they need to live healthier lives and ensure that future generations live free of preventable diseases like polio.”

Partners hope that The Big Catch-up will help lift vaccination levels among children to at least pre-pandemic levels.

During the pandemic, health services got overwhelmed, some clinics closed and the imports and exports of vials and syringes plus other medical supplies was disrupted.

Lockdowns also restricted travel and access to services.

In Uganda, travelling was restricted during the coronavirus lockdown

Other factors also caused a decline in immunisation: conflicts, climate crises and vaccine hesistancy.

Everyone is now being rallied to their part in helping to catch up by reaching the children who missed out during the pandemic.