Longer Delay Recommended Between Covid-19 Vaccine Doses

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(ADB/Creative Commons)

Recent research and manufacturer guidelines say some of the Covid-19 vaccines available in Cambodia are likely more effective when individuals wait longer for their second shots, as vaccines have been distributed in some cases at an interval as short as two weeks.

The question of the optimal duration between shots comes at a time when Cambodia is experiencing an urgent outbreak, while distributing the 1.5 million doses of the Chinese-produced Sinovac vaccine, which arrived in late March. The country has also received 1.3 million doses of the Sinopharm vaccine from China, along with 324,000 doses of AstraZeneca’s Covishield via the Covax initiative. The goal is to vaccinate 10 million total individuals.

Health Ministry spokeswoman Or Vandine stated in a voice audio message on April 9 that for Sinovac and Sinopharm, the second dose should be taken between 14 to 28 days after the first. She did not reply to questions about discrepancies in research and recommendations.

The director of Phnom Penh’s National Pediatric Hospital, Nhep Angkeabos, said his hospital was providing Sinovac to civil servants on a flexible timescale, with the second dose possible as early as two weeks after the first.

“For the second dose of Sinovac and Sinopharm, [dose intervals for] both are the same — between two weeks to four weeks — but if people come on the third week it is better than two weeks,” Angkeabos said.

“This is based on a test from Chinese laboratory,” he added. “We just followed the laboratory and the medicine producing factory. We just follow their advice.”

Li Ailan, World Health Organization representative to Cambodia, said Sinovac currently recommends that the second dose of its vaccine be delivered after 30 days “in routine settings,” though the second dose can be given after two weeks “in an emergency context.” She added that Sinopharm advises 21 to 28 days between doses.

Li said the WHO will not issue its own recommendation on dose intervals for Sinopharm or Sinovac until the agency has finished reviewing the vaccines. In late March, the WHO stated that the vaccines were showing promising signs of efficacy and safety and expected to issue a decision by the end of April.

Publicly available research suggests that a longer interval could produce more effective antibody protection.

A study published in the medical journal The Lancet in February found that more antibody protection was generated after Sinovac was given 28 days apart instead of with a 14-day interval.

Likewise, a Brazilian study released in January found that individuals who received their Sinovac vaccine three weeks apart had an efficacy rate of nearly 70 percent. But for another group of volunteers, most of whom received the vaccine 14 days apart, that efficacy rate dropped to 50.4 percent. More recent data from the University of Chile last week shows Sinovac as 54 percent effective in reducing infection.

Other governments relying on the Chinese-produced vaccines are mandating more than two weeks between doses.

In the Philippines, which has been topping ten thousand daily cases, the Department of Health states that Sinovac doses should be given four weeks apart. The same interval is applied in Hong Kong.

In the United Arab Emirates, one of the first countries to employ Sinopharm, the vaccine doses are distributed on a three to four week interval.

However, at least some health officers in Cambodia are directing those receiving their first shot of Sinovac to return after just two weeks.

After visiting a health center in Siem Reap last week for her first shot of Sinovac, one woman showed VOD that the date for her second shot 14 days later had already been written onto her vaccination card. Though the health center, which declined to comment, offered the option for her to return three or four weeks later, the woman said she would come sooner.

“The date that they wrote for us to go back to take the second shot is after 14 days,” said the woman, who requested anonymity. “All of us will go [on] the date that they told us.”

VOD staff who received their first Sinovac shots at the Information Ministry in Phnom Penh on April 12 were asked to return on April 27.

In other cases, such as for Prime Minister Hun Sen’s son Hun Manet, who received the first official shot of the nationwide vaccination campaign on February 10, individuals have received Sinopharm doses three weeks apart.

A Sinovac spokesman told the AP this week that the vaccine’s protection “may be better” if the second dose is given more than 14 days after the first, but did not clarify further.

The director of the Chinese Center for Disease Control and Prevention also recently acknowledged that the current Chinese vaccines “don’t have very high protection rates” and is considering ways to enhance their effectiveness.

Unicef Cambodia — assisting with the vaccine rollout in the Kingdom — states on its website that “based on current information, most vaccines require two doses, with the second dose administered a few weeks after the first dose.” A Unicef spokesperson deferred to the WHO and did not specify which vaccines were indicated in the statement.

As for those receiving the WHO-approved Covishield by Oxford-Astrazeneca, the Ministry of Health initially recommended a four to eight week interval between shots but extended this time to eight to 12 weeks following a recommendation from the WHO last week. AstraZeneca has also said its vaccine is more effective with longer dose intervals.

The initial shortened interval had likely been to ensure that the Covishield vaccines given to Cambodia by the Covax facility could all be used before their expiration date in late June, health officials explained.

“With the new recommendations, if we vaccinate from eight weeks up it is more effective,” said Kandal Provincial health department director Kuoy Buntheoun. “The protection is better.”

Li, the WHO representative to Cambodia, stressed that while vaccines are “a critical new tool in the fight against COVID-19” none are 100 percent effective, regardless of dose interval. She urged continued vigilance in following protective measures such as mask-wearing, social distancing and avoiding confined spaces.

“The recommended dose interval for different COVID-19 vaccine products may change in the future as studies continue and we learn more about the vaccines,” she said.

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