The government starts administering the second shot of AstraZeneca's Covid-19 vaccine today amid fast-depleting stocks and a strained supply chain.

Experts say a second shot of the vaccine can ensure more consistent protection. Efficacy of the vaccine reaches 82 percent after the second dose from 76 percent after the first, according to AstraZeneca.

People who got the first jab during the piloting of the inoculation campaign on January 27-28 and 31,160 others who got their first shot on February 7, the first day of the mass inoculation campaign, are likely to get their second shots today.

"The second dose will make more people safer, but we have to follow the health rules nevertheless," renowned virologist Prof Nazrul Islam told The Daily Star yesterday.

The former vice chancellor of Bangabandhu Sheikh Mujib Medical University also suggested having antibody tests to assess the state of antibodies in people after inoculation.

Prof Sayedur Rahman, chairman of pharmacology at BSMMU, said, "The second dose boosts the immune system to produce enough antibodies. It also develops a memory cell response which lasts for a long time."

Over 55 lakh people, which is about 1.7 percent of the country's population of 170 million, have received the first shot.

The government currently has around 45 lakh doses in stock to simultaneously administer the first and second doses. Experts say it's not enough for an uninterrupted inoculation campaign.

Besides, there is an uncertainty over future shipments from the Serum Institute of India which has failed to ship the third consignment of 50 lakh doses of the AstraZeneca vaccine in the last week of March.

Bangladesh purchased three crore doses of the AstraZeneca vaccine from Serum and has so far received only 70 lakh doses in two instalments. According to the agreement, Bangladesh was supposed to get 1.5 core doses in monthly shipments of 50 lakh doses from January.

The country also received 32 lakh doses in two instalments as gifts from the Indian government. The uncertainty over the new shipment was also evidenced in a recent statement of Serum CEO Adar Poonawalla.

Serum will be able to resume vaccine export if the second wave of Covid-19 in India is checked by June-July, Poonawalla was quoted as saying, reports our correspondent in New Delhi.

Serum's present manufacturing capacity is "very strained" and has put the company's commercial supply commitment to other countries under stress, Poonawalla said on Tuesday night.

Serum produces 60-65 million doses of the vaccine named Covishield every month, he said, adding that the firm supplied around 100 million doses to the Indian government and exported 60 million doses. But the company is "still short of being able to supply to every Indian" who needs the vaccine.

Poonawalla also said ramping up vaccine production could take nearly three months.

The delay is likely to keep Bangladesh from getting on time the doses from the Covax, which has committed to give 6.8 crore shots to the country.

The country is supposed to receive 10,908,000 shots by May, as per that commitment.

Contacted, Health Minister Zahid Maleque said vaccination would continue as long as there are doses in stock.

"But Serum's commitment needs to be fulfilled by this month if we are to run the vaccination campaign smoothly. Otherwise, it will stumble," he said, adding that the ministry was constantly in touch with Beximco Pharmaceuticals, the local agent of Serum.

He hopes that the next shipment will arrive soon.

Beximco Managing Director Nazmul Hassan Papon said he was expecting an update about the shipment next week.

"We hope we will get the details from Serum shortly."

Officials said the government was exploring alternative sources for the vaccine. The health ministry  has written to AstraZeneca to either give Bangladesh the vaccine seed or the doses in bulk so that it can be produced or put in ampules locally.

The government also talked to the Chinese and Russian ambassadors in Dhaka to purchase the vaccine called BBIBP-CorV and Sputnik V respectively.

As of yesterday, the government did not get any response from any of the sources.

Pharmaceutical companies in Bangladesh are also communicating with vaccine inventors to be able to produce the shots in the country.