According to UNICEF, vaccination coverage dropped sharply during the COVID-19 pandemic.
The UN Children's Fund (UNICEF) says some 67 million children globally missed out entirely or partially on routine immunisation from 2019 to 2021 due to COVID-19.
The organisation said this in its "State of the World's Children 2023' report.
According to the Fund, vaccination coverage dropped sharply during the COVID-19 pandemic, leaving millions more children unprotected against some of childhood's most serious diseases.
It added that the pandemic had been a disaster for childhood immunisation as it set immunisation back to levels last seen in 2008 in those two years, as the world lost more than a decade of progress in ensuring every child was adequately immunised.
The report said that between 2019 and 2021, the number of zero-dose children rose from 13 million to 18 million globally, an increase of more than a third.
It also said that there was a sharp increase in the number of under-vaccinated children, which rose by six million to 25 million.
It added that "this backsliding reflected some issues specific to the pandemic, particularly the impact of lockdowns and service disruption.
"However, it also cast a powerful spotlight on longer-term issues, including the weakness of far too many primary health care systems, which has long undermined efforts to vaccinate every child."
The organisation identified issues that led to the decline, including strains on overstretched health systems and health workers, especially overworked women health workers, as well as confusing communication with parents.
It stated that "perhaps the most significant factor was the impact of the pandemic on primary health care and health systems.
"As previous crises have shown, countries with already weak health systems are especially vulnerable to the impacts of conflict, major disease outbreaks and natural disasters.
"The COVID-19 pandemic was no exception, as it forced many health systems to divert scarce resources away from providing routine care, including immunisation."
It added that at the end of 2021, nearly half of 72 countries in a World Health Organisation (WHO) survey said that routine vaccination programmes were being disrupted by the need to respond to the pandemic.
Also, the pandemic badly interrupted vaccination campaigns, which, unlike routine immunisation, usually targeted efforts to vaccinate large numbers of people in a relatively short period of time.
Citing an example, it said that in May 2020, 57 per cent of campaigns in 57 countries had to be cancelled or postponed, representing the loss of 796 million vaccine doses.
Although the situation started improving in mid-2020, disruptions continued and at the end of 2021, stalled campaigns in African countries, causing the loss of 382 million doses.
The impact on the health workforce was also felt as even before the pandemic, many countries were short of skilled health workers, particularly in disadvantaged areas.
The pandemic intensified the problem and added to the challenges facing health workers, especially women in the health workforce, the report said.
The report stated that "a final factor is the impact the pandemic had on families' abilities and willingness to get their children vaccinated.
"Even where healthcare facilities remained open, travel restrictions or tight family budgets may have led families to put off getting children vaccinated.
"Stay-at-home' recommendations may have led some parents to see routine vaccination as non-essential care, which underlines the need for careful, nuanced communication with families during major disease outbreaks.
"Significantly, parents may also have been wary of visiting clinics for fear of contracting COVID-19."
UNICEF notes that catching up on the children who missed out entirely or partly on vaccination during the pandemic will be a major challenge and will require substantial investment to design and implement appropriate catch-up interventions.
In the face of difficult economic headwinds, there is also a need to support health and immunisation services to prevent continued backsliding, the organisation said.
On catching up and recovery, UNICEF said investments in areas such as primary healthcare, vaccine development and delivery, and innovations are required to reach global immunisation goals.
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According to the UNICEF report, there is an urgent need to reach the children who missed out on vaccination because of the pandemic and other factors, including conflict, with intensified catch-up initiatives.
These initiatives, it added, would need to identify and locate zero-dose and under-vaccinated children and missed communities as they would allow the development of specific plans and strategies to ensure catch-up initiatives reach the communities and children with the greatest needs.
It, however, said that catch-up alone would not be enough, recommending countries where the pace of recovery in immunisation services is slow to ensure full restoration to at least pre-pandemic levels as quickly as possible.
UNICEF added that "calamitous as it has been, the COVID-19 pandemic brought changes in the vaccine landscape that the global community must capitalise on to boost childhood immunisation.
"The experience of the pandemic revealed that, with political will and leadership, vast resources can be mobilised, and new vaccines can be developed rapidly and introduced around the world."
