The U.S. should postpone second doses of the Pfizer/BioNTech COVID-19 vaccine in order to conserve supply and inoculate as many people as possible, two Canadian researchers recommended, citing data showing the initial dose to be highly effective.

The two researchers — Danuta Skowronski at the British Columbia Centre for Disease Control and Gaston De Serres at the Institut National de Santé Publique de Québec — pored over the trial data submitted to the FDA and found that the Pfizer vaccine was highly effective even before the second dose.

According to their analysis, the vaccine showed 68.5 percent efficacy a week after inoculation with the first dose, but starting two weeks after the initial shot, its efficacy ramped up to 92.6 percent.

In a letter to the New England Journal of Medicine (NEJM), the two urged the U.S. to begin conserving second dose Pfizer shots and administering as many initial doses as possible to help prevent winter hospitalizations and deaths.

“With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose,” they said.

They cautioned that questions still remain about how long the protection from a single dose lasts, but contended that administering the second dose shot as currently recommended “provides little added benefit in the short term, while high-risk persons who could have received a first dose with that vaccine supply are left completely unprotected.”

So far, the FDA has stuck to a Jan. 4 statement on the authorized Pfizer and Moderna vaccine dosing schedules, stating that the currently available data support administering Pfizer’s vaccine doses 21 days apart and Moderna’s 28 days apart.

The agency cautioned that first-dose data is being misinterpreted, noting that in Pfizer’s phase 3 trial, 98 percent of participants received two doses of the vaccine at the three-week mark. Those who did not receive the vaccine at that interval were generally only tracked for a short time, “such that we cannot conclude anything definitive about the depth or duration of protection after a single dose.

In a separate letter to the NEJM, researchers from Pfizer Vaccine Research and Development and the University of Texas Medical Branch outlined their findings from a laboratory study suggesting that the Pfizer vaccine proved less effective against the South African coronavirus strain. For their study, the researchers used genetically engineered viruses and serum obtained from vaccinated individuals.

Peter English, former chair of the British Medical Association’s Public Health Medicine Committee, cautioned that the study only tested for the number of neutralizing antibodies and he did not find the results conclusive.

“This adds a little to the information we have; but we must remember that these are ‘in vitro’ findings. They are only what happens when you mix serum and antigens in the laboratory; and they only test the quantity of neutralizing antibodies,” he said.

“We know that other aspects of the immune response — including cellular immunity (memory cells, killer T-cells and so on) — are also important in the response to this virus. While these findings might, possibly, suggest that the vaccine will be less effective against the South Africa B.1.351 variant, this remains only a possibility — it is far from proven.”

Read the Canadian researchers’ letter here: bit.ly/3u9FP67.

Read the Pfizer/University of Texas letter here: bit.ly/37rQ2RB. — James Miessler