The shortages, caused partly by manufacturing delays and Indian supply disruptions, come as cases and deaths increase across Africa as part of a third wave of infections.
The WHO said last month it was reviewing a proposal by an unidentified vaccine manufacturer in Vietnam to become an mRNA-based COVID-19 vaccine technology hub in the Southeast Asian country.
"We are working hard with AstraZeneca and with our scheduling and we are about to reschedule about 16 million doses to try and cover those second doses" to be shipped out in late June and early July, he said.
Iran, the Middle Eastern country by COVID-19, has complained that US sanctions were preventing it from making payments to buy vaccines.
Ardakanian said the payments would go towards the purchase of vaccines from the World Health Organization (WHO) - sponsored global COVAX vaccine-sharing plan, state news agency IRNA reported.
"While we appreciate the work of AstraZeneca who have been steadily increasing the speed and volume of their deliveries, we need other manufacturers to follow suit," said WHO Director-General Tedros Adhanom Ghebreyesus at a virtual briefing, mentioning Pfizer and Moderna specifically.
Despite being hit by reduced supply availability in March and April, and higher demand in India where much of its main jab AstraZeneca is made, it expects to deliver doses to all economies that have requested them in the first half of 2021, the GAVI vaccine alliance and World Health Organization said.
COVAX, backed by WHO and the Global Alliance for Vaccines and Immunization, aims to secure 2 billion vaccine doses by the end of 2021.
In early March, it said the target was to deliver 237 million doses of AstraZeneca's shot to 142 countries by the end of May, and it also shipped its first Pfizer shots.
I feel so good about taking the vaccine. It will protect me from contracting the virus from patients.
COVAX aims to deliver over 1.3 billion vaccine doses to over 90 low- and middle-income countries by the end of the year, covering up to 20% of their populations.
J&J said an emergency use listing is a prerequisite for supplying vaccines to the COVAX vaccine program, co-led by WHO, which aims to deliver doses to poor and middle-income countries.
The J&J vaccine is administered in a single dose and can be stored at normal fridge temperatures, a big selling point in countries with relatively weaker healthcare infrastructure.
The COVID-19 vaccine developed by Oxford University and AstraZeneca gives good immune responses in older people.
The EMA has actually not restricted this vaccine in terms of age based upon the entire data packages, and of course that is a stringent regulatory authority.
Our hope is this will lead to consensus to keep vast majority of work done through multilateral mechanisms and not bilateral. We have no ability to stop those bilateral mechanisms.
Covax has two price points for the Astrazeneca vaccine, $3 and $4 a dose, depending on the country, he said.
Tedros Adhanom Ghebreyesus, WHO director-general, appealed for $4 billion to buy COVID-19 vaccines for distribution in lower and middle-income countries.
Vaccines offer great hope to turn the tide of the pandemic. But to protect the world, we must ensure that all people at risk everywhere - not just in countries who can afford vaccines - are immunised.