Health officials are intensifying their calls for all American adults to get booster shots amid the threat of the omicron variant, a strain of COVID-19 first discovered in South Africa. Data released by Pfizer this week showed a significant drop in the antibody response to the omicron strain with two doses, but that response, according to the pharmaceutical company, was restored with a third dose. Even before the discovery of omicron, many experts pointed to evidence of waning immunity over time from two doses…