How efficacious?

Published January 12, 2021, 12:19 AM

by Jullie Y. Daza


Jullie Y. Daza

“If you get vaccinated with a dose that’s 50 percent effective, it means you reduce the infection rate by 50 percent.” It does not mean that the vaccine is only half good. If the vaccine is 95 percent efficacious, “you’re 95 percent sure of protection.”

Edson Guido, head of ABS-CBN data analytics, explains: “A 50 percent efficacy corresponds to 50 percent of risk reduction. It does not mean that half of those who were inoculated won’t get covid while the other half will. The efficacy rate is always relative to the risk of those who are exposed to the virus.”

After Pfizer’s claim of a 95 percent efficacy and Moderna’s 94 percent, compared with Sinovac’s 50 percent that was “acceptable” to the Department of Science and Technology, we find that there’s much to learn about a virus that’s still as mysterious as it is invisible. It would help if infectious-disease experts could talk our language.

Edson, who’s not an M.D. but an interpreter of data for the layman, continues: “A 50 percent efficacy rate indicates that a person is 50 percent less likely to be infected” than someone who has not been vaccinated. Taking the positivity rate of 5 percent (as reported in the last two weeks in the Philippines), “the risk of getting covid for those who were injected with a vaccine of 50 percent efficacy is reduced to 2.5 percent” (half of 5 percent).

To put it another way, “a 50 percent efficacy does not mean the vaccine works only half the time.” Rather, it means that “the vaccine halved the chances of people who were vaccinated from becoming sick relative to those who did not.”

Among other issues confronting the nation, apart from the obvious one of why the “czars” are moving with glacial speed to procure the vaccines, are the extremely high cost of the anti-covid infrastructure, and ensuring that those who have been vaccinated get their second shot.

Now that Red Cross has confirmed that a saliva test is available to replace those expensive,  uncomfortable RT-PCR tests, the guessing game – how much, $2.50, $5, P1,000? — is centered on this question: Cheaper, and just as good if not better, with results assured after three hours, what are we waiting for?