Don’t skip that second dose of the Pfizer or Moderna vaccine, even if you’ve had previous exposure to COVID-19.

A new study suggests that people who experienced asymptomatic or mild cases of COVID-19 don’t have a greater level of immunity to the virus after their first vaccine dose compared to those who never tested positive for the virus.

Northwestern University researchers analyzed antibody reaction in blood samples from 27 Chicago-area vaccinated adults, some of whom previously tested positive for COVID-19, two weeks after their first dose, three weeks after their second dose and approximately two months following the second dose.

Northwestern professor and biological anthropologist Thomas McDade, the study’s lead author, said two weeks after the first vaccine dose, the antibody response of participants who previously experienced asymptomatic or mild cases of COVID-19 was virtually the same as participants who weren’t previously exposed to the virus.

“These mild and asymptomatic cases do not generate robust antibody responses and probably don’t result in a high level of protection against reinfection,” McDade said. “This has implications for vaccination as well, because we see that people who had mild or asymptomatic infections do not respond strongly to the first dose of the two-dose vaccines—they need two doses to reach full protection.”

The study found participants’ antibody responses decreased by about 20% two months after their second dose – although specific antibody response depended on prior exposure to the virus. McDade said participants with histories of more severe, symptomatic COVID-19 cases had a higher level of antibodies.

McDade said their findings support the need for everyone to get fully vaccinated, even if antibody reaction wanes over time and the current vaccines are less effective against emerging variants of the virus, like the delta variant.

“Based on our research, and research of other groups, the vaccines are very effective at generating immune protection against the variants, even if that level of protection isn’t quite as high as it would be against the original version of the virus,” McDade said. “This is part of the reason why we are seeing more cases of mild breakthrough infections. But protection against serious COVID remains very high.”

Read the full study, published Aug. 30 in the journal Scientific Reports.