Large cities in Brazil have been forced to discontinue administering second doses of COVID vaccine due to lack of supply, marking another hurdle in the country's vaccination program.

Many of Brazil's 26 state capitals have stopped giving the booster shot of CoronoVac, a Chinese-developed vaccine.

Rio de Janeiro has already announced on Saturday that it would stop second doses of the vaccine for doses. This was before changing its immunization calendar to ensure that vaccine shots would be given by age group, according to an AFP News report.

Those aged under 60, including health workers, could have to wait two to 12 days beyond the recommended 28-day interval between doses.

READ MORE: Brazil Now No. 2 in World's COVID-19 Cases Surpassing Russia

Brazil's COVID Vaccination Program

Brazil's Health Ministry said that around 1.5 million people were missing their vaccine appointments for the second dose.

Experts expressed their concern after a recent real-world study from Chile found that Sinovac Biotech is just 16 percent effective after one shot.

Sinovac accounts for some 80 percent of the country's vaccination program.

"Without the two doses, we get neither full protection nor a long duration of protection," Juarez Cunha, head of the Brazilian Society of Immunizations, was quoted in a Reuters report.

Cunha noted that they need people to do the full cycle of the vaccine.

The country's vaccination program has missed its targets due to a shortage of doses due to delivery delays of active ingredients from China and India.

Experts and officials involved in the program say, aside from the shortage, poor communication is also one of the factors why the program is missing its target.

They also consider people's lack of knowledge on the importance of getting the second shot, adding that sometimes people do not remember their schedules.

It can also be attributed to people being put off by a strong reaction to the first dose, which are short-lived fever and body aches.

In addition, there have also been long lines at some vaccine sites.

Brazil's Health Ministry said that it is preparing a national media campaign to raise awareness on the significance of completing the vaccination cycle.

Meanwhile, the country had also disapproved Russian COVID vaccine Sputnik V for the time being after it found that the vector virus Adenovirus 5 is not inactive and can multiply.

This could be dangerous, particularly for people with a weak immune system, as the cold virus used as a carrier was not inactivated, according to a DW report.

Slovakia had also stopped the delivery of Sputnik V after they found that 200,000 vaccine doses delivered were not identical to the Sputnik V vaccine used in other countries.

Slovak regulators said that the vaccine has only a name in common. Russia was quick to dispute the claim, saying that it was fake news.

Latest reports from Brazil were also dubbed by Russia as fake news. The Gamaleya Institute stated that reports of alleged failure to inactivate the vector virus are a disinformation campaign.

The vaccine manufacturer also said that Brazil's decision to stop using Sputnik V was political.

READ ALSO: Brazil Health Regulator Rejects Russia's COVID Vaccine

WATCH: Brazil's remote Amazon communities receive Covid vaccines - from BBC News