Concerns over waning immunity and SARS-CoV-2 variants have convinced some countries to deploy extra vaccine doses, but it’s not clear to scientists whether most people need them.
A study published in the New England Journal of Medicine on August 12, 2021, evaluated the effectiveness of 2 Covid-19 vaccines (Pfizer and AstraZeneca) against symptomatic disease by the B.1.617.2 (Delta) variant. They examined data from symptomatic persons 16 years of age or older who underwent Covid-19 testing in England between October 2020 and May 2021, and assessed vaccination status on persons who tested positive for the delta variant (4,272 persons) as well as people positive to the alpha variant (14,837 persons).
They found that both vaccines were highly effective against the delta variant. The Pfizer vaccine showed a decrease from 93.7%-88% and the AstraZeneca vaccine from 74.5%-67% against symptomatic disease.
The question has been raised among several countries due to the soaring infecion numbers caused by the highly contagious Delta variant of SARS-CoV-2.
How do boosters work?
Vaccination produces an initial surge in the number of immune cells that produce antibodies and other molecules, which then slowly drops. The type of cells that produce antibodies are the plasma cells that at a later stage in immunity can become memory cells and secrete antibodies for long lasting protection, these cells can be maintained for decades or even a lifetime in the bone marrow. There is also the T cell immunity which is harder to evaluate.
A booster does different things to these cells. It causes antibody-making B cells to multiply, elevating the levels of antibodies against the pathogen once more. In time, their numbers will dwindle again, but the pool of memory B cells left behind will be larger than before, leading to a faster and stronger response to subsequent exposures. They also promote something called affinity maturation in which engaged B cells, those triggered by the vaccine, travel to the lymph nodes and gain mutations making the antibodies they produce bind to pathogens more strongly, potentially enhancing their potency.
A few trials have tested extra doses and support this. They have shown a drop over time in antibody titres and T-cell titres that is expected like with other vaccines. What scientists don’t know is whether these drops reflect a decline in protection against the virus. Teams around the world are racing to determine what level of neutralizing antibodies or another immune marker is most closely associated with a vaccine’s effectiveness, that is known as correlate of protection.
A study performed by the Ministry of Health in Israel, a country that has one of the world’s highest vaccination rates, released raw data on vaccination and infections from December 2020 to July 2021. They estimated that vaccine protection against both infection and disease had dropped from above 90%-40%. They said that the date could have biases due to some variables. One of them is the fact that a lot of the infections were on health care workers who were vaccinated early on and that are at a higher risk of infection than younger people vaccinated later. Early-vaccinated individuals also tended to be wealthier than people who got vaccinated later in Israel and might have taken COVID-19 tests at a higher rate because of worries over the virus or to travel internationally.
Something that is clear is that protection against severe disease remains high. Pfizer- BioNtech and Moderna reported percentage efficacy estimates in the high 90s against severe COVID-19 after 6 months. Some countries like the US are reporting that up to 99% of the people experiencing severe illness from COVID-19 are unvaccinated.
Some scientists are recommending vaccine boosters for immunocompromised and cancer patients or those older than 60 years. More studies need to be conducted before a recommendation of boosters for all the population is made and with only 30.7% of the world population that has received at least one dose and even less, only 16% being fully vaccinated, there is a need to first finish the vaccination of the majority of the population. That is why on August 4 the WHO called for a moratorium on boosters until at least the end of September.
Jamie Lopez Bernal, et al. Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant. August 12, 2021. N Engl J Med 2021; 385:585-594.
Source link: https://www.nature.com/articles/d41586-021-02158-6