Tulane University virologist Robert Garry points out that the flu vaccine, for instance, doesn’t give absolute protection against influenza, but rather is designed to prevent a significant infection and keep things “from falling off a cliff.” A preprint study uploaded this month, which measured antibody levels in patients in London, found that between 2% and 8.5% didn’t even develop detectable antibodies. An infection that causes severe symptoms is likely to lead to a stronger immune response, which would also help encourage strong and longer-lasting immunity moving forward. On the flip side, a mild or asymptomatic case is likely to yield lower antibody levels, as was found in covid-19 patients in a new study published in Nature Medicine on Thursday. In the study, researchers studied a small cohort of asymptomatic patients with covid-19 and found that they seemed to develop lower antibody levels. We’re still not totally clear how asymptomatic cases and symptomatic cases differ in terms of immunity, since the asymptomatic cases aren’t being actively tested and identified. Fortune says that for covid, we’re likely to get true protection, but we don’t know for sure—and it’s not something that we can just assess from antibody levels. It won’t be until phase III trials (which will directly measure the vaccine’s efficacy) that we’ll have a better sense of what the relationship between antibody levels and immunity is, and what sort of immune response a vaccine needs to elicit to provide true protection.