On 28 July 2021, Pfizer and BioNTech posted updated results for their ongoing phase 3 covid-19 vaccine trial. The preprint came almost a year to the day after the historical trial commenced, and nearly four months since the companies announced vaccine efficacy estimates "up to six months.". But you won't find 10 month follow-up data here. While the preprint is new, the results it contains aren't particularly up to date. Since late last year, we've heard that Pfizer and Moderna's vaccines are "95% effective." Measuring vaccine efficacy two months after dosing says little about just how long vaccine-induced immunity will last. "Waning immunity" is a known problem for influenza vaccines, with some studies showing near zero effectiveness after just three months. And so the recent reports from Israel's Ministry of Health caught my eye. In early July, they reported that efficacy against infection and symptomatic disease "fell to 64%." By late July it had fallen to 39% where Delta is the dominant strain. This is very low. For context, the FDA's expectation is of "at least 50%" efficacy for any approvable vaccine. Now Israel, which almost exclusively used Pfizer vaccine, has begun administering a third "booster" dose to all adults over 40. The US plans to follow suit. Until new clinical trials demonstrate that boosters increase efficacy above 50%, without increasing serious adverse events, it is unclear whether the 2-dose series would even meet the FDA's approval standard at six or nine months.