New York, Oct 5 : Two doses of Pfizer-BioNTech (BNT162b2) are 90 per cent effective against Covid-19 hospitalisations for all variants, including delta, for at least six months, confirms a new study published in The Lancet
However, effectiveness against all SARS-COV-2 infections declined, falling from 88 per cent within one month after receiving two vaccine doses to 47 after six months, stressing the need for a booster dose.
The joint study by Kaiser Permanente Southern California and Pfizer underscores the importance of improving Covid-19 vaccination rates worldwide and monitoring vaccine effectiveness to determine which populations should be prioritised to receive booster shots.
"In line with the recent FDA and CDC recommendations, considerations for booster shots should take global Covid-19 vaccine supply into account as people in many countries around the world have not yet received a primary vaccination series," said lead author Dr Sara Tartof from Kaiser's Department of Research and Evaluation.
The team analysed 34,36,957 electronic health records between December 4, 2020, and August 8, 2021, to assess BNT162b2 vaccine effectiveness against SARS-CoV-2 infections and Covid-19-related hospitalisation.
During the study period, 5.4 per cent of people were infected with SARS-CoV-2.
Among those who were infected, 6.6 per cent were hospitalised. The average time since being fully vaccinated was between three to four months.
Vaccine effectiveness against delta variant infections at one month after two doses of BNT162B2 was 93 per cent and fell to 53 per cent after four months.
Effectiveness against other (non-delta) variants at one month after receiving two doses was 97 per cent and declined to 67 per cent after four months.
Effectiveness against delta-related hospitalisations remained high (93 per cent) for the duration of the study period.
Researchers did not observe a difference in waning between SAR-CoV-2 variant types.
However, the authors note that because delta became the dominant strain in the middle of the study period, analyses with longer follow-up to measure the rate of waning for delta compared to other variants are warranted.
"Covid-19 infections in people who have received two vaccine doses are most likely due to waning and not caused by delta or other variants escaping vaccine protection," said Dr Luis Jodar, Senior Vice President and Chief Medical Officer, Pfizer Vaccines.