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What Does Vaccine Efficacy Mean?

Math is simple, decisions not so much.
Mar 1, 2021
Photo/s: Jerome Ascaño

The Philippines on March 1 delivered its first COVID-19 vaccines using CoronaVac from China's Sinovac. There's some skepticism online as the jab has an efficacy rate of 50.4% and clinical trials have been largely shrouded in secrecy.

CoronaVac's 50.4% efficacy rate compares with 95% for the Pfizer BioNTech vaccine and 70% for AstraZeneca.

Simply put, the efficacy rate measures by how much a vaccine reduces the risk of getting COVID-19 in a population. The higher the efficacy rate, the lesser the risk of people getting sick from a virus, in this case, SARS-CoV-2.

In soothing vaccine doubts, Health Sec. Francisco Duque III assured the public that jabs approved by the Food and Drug Administration offer 100% protection against severe COVID-19.


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Consider a population of 100

Eloisa Lopez, Reuters
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If for example, a vaccine with 50% efficacy is given to a population of 100, half will likely get sick while the other half will not get sick, according to Dr. Edsel Salvanawho heads the Institute of Molecular Biology and Biotechnology at the National Institute of Health.

Given that the 100 were vaccinated, those who do get sick will be less likely to infect others since they were inoculated, said Salvana, who is also part of the Technical Working Group on COVID vaccines.

Efficacy rate is based on trials

Jerome Ascaño
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It is important to note that the efficacy rates are based on controlled human trials. At least for now. Vaccine trials take a representative sample to gauge its effects on the larger population. It's like an opinion poll.

Vaccine efficacy is among the top criteria for approval of any jab, according to infectious disease expert Dr. Nina Gloriani, who heads the technical group. Before COVID, Gloriani had overseen numerous vaccine trials, including one for leptospirosis.

So far, only the Pfizer vaccine has had a real-world efficacy that came close to the trial-based efficacy -- 94% based on the Israel vaccine rollout.

Real world is different

Eloisa Lopez, Reuters
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Outside of the laboratory, optimum vaccine efficacy rests on several factors, said the Canadian Center for Vaccinology, which was published by the World Health Organization.

Foremost is the condition of the primary care center where the vaccine is delivered. Some vaccines for example require storage at sub-zero temperatures. The Sinovac jab doesn't require deep freezing and during the ceremonial inoculations on in Manila on March 1, vaccinators only needed gloved hands to handle the doses.

The health of the recipient also plays a part. It will depend on whether they have co-morbidities, even age, according to the WHO paper. In Norway, there were initial concerns over senior citizens dying after getting the Pfizer jab. Authorities later said the deaths were not directly related to the vaccine.

How vaccines fight the virus

Jerome Ascaño
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At least in the case of Sinovac Salvana illustrated efficacy this way: It will reduce a lion into a cat.

"Yung leon po, ginagawa nating kuting. We are turning COVID-19 from a deadly disease that shut down the whole world into a common cold," he said.

"Wala pong takot sa sipon. Madami pong takot sa COVID. Yan ang ginagawa natin, tatanggalan natin ng pangil ang COVID para hindi na siya nakakatakot," he said.

Should you wait for a vaccine with higher efficacy?

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Government officials are urging the public to grab the first available vaccine. The benefits of getting protected now far outweigh waiting and leaving oneself vulnerable to COVID.

“Nakadesisyon po tayo sa Sinovac base sa siyensya. Sinabi ko po sa town hall, alisin natin ang ating view sa vaccine, hiwalay sa pulitika, hiwala sa partido, hiwalay sa administrasyon, hiwalay sa paniniwala kundi batay sa siyensya na malinaw,” said the first Filipino to get the official vaccine, Philippine General Hospital Director Dr. Gerardo Legaspi.

Some healthcare workers have protested the use of Sinovac saying they were promised the Pfizer jab.

"The best vaccine is the vaccine that's in your arm," said Salvana. "Tanggapin po natin kung anong bakuna nandyan dahil po ito ay makakasalba ng buhay."

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