New York, Feb 26 (IANS): The effectiveness of the Covid-19 vaccine may substantially decline after six months if booster shots are not taken, according to a new study published in The Lancet Respiratory Medicine.

The study indicated that vaccines were 94 per cent effective at preventing hospitalisation 50-100 days after receiving the shot but fell to 80.4 per cent 200-250 days later, with even more rapid declines after 250 days.

"This data helps us understand differences in waning protection by vaccine type and identify the key risk factors for severe breakthrough infections to help inform the targeting of potential vaccine booster programmes," said Amy Compton-Phillips, Providence chief clinical officer.

"Unlike most other studies, our data stretched beyond six months, where we found evidence of rapidly waning protection, especially for patients 80 or older," Compton-Phillips added.

In addition to examining the effectiveness of vaccines over time, the Providence study was also able to identify factors associated with reduced vaccine effectiveness.

Key risk factors for a severe "breakthrough" infection included advanced age (80+), comorbidities such as cancer, transplants, chronic kidney disease, hypertension, or heart failure, the amount of time that had elapsed since being vaccinated, and the type of vaccine one received.

For the latter factor, the study found that the Moderna vaccine offered the best overall protection over time, while the Pfizer-BioNTech vaccine offered initial protection equivalent to Moderna's but declined more rapidly over time.

Persons receiving the Janssen vaccine also had higher odds of experiencing a severe breakthrough infection compared to Moderna.

For the study, the research team examined data from nearly 50,000 hospital admissions between April and November of 2021.