Coronavirus disease 2019 (COVID-19) proved extremely difficult to control once it entered a country – while there were a few success stories such as New Zealand, most countries ended up resorting to lengthy and economically painful lockdowns, among other restriction measures.
Even these did not fully control the spread of the disease until vaccines were developed and mass vaccination programs began to take shape.
Vaccines
Traditionally, vaccines are created from an inactivated or attenuated form of the virus. This allows the antigens to be presented to the immune system in a mostly safe way, making specific antibodies prior to any potential 'real' infection.
While this is effective, there have been concerns over safety, as inactivated viruses can become active again, or attenuated strains could receive new traits.
mRNA
The new messenger RNA (mRNA) vaccines avoid such risks, as no structural proteins are present. In fact, no virus proteins are present in the mRNA vaccines at all. They contain a small string of mRNA encoding for a particular antigen of the virus and rely on the cells' own machinery to transcribe it. Thus, the immune system can become familiarised with the virus with no risk.
In order to investigate the effectiveness of these vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19 disease, a group of researchers from the Instituto Nacional de Saude Doutor Ricardo Jorge in Portugal have been examining electronic health registries of the elderly. Their work is reported in the journal Eurosurveillance as a Rapid Communication.
The researchers excluded those in care homes and those previously infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Previous studies have shown that antibody retention and immunity increase dramatically with previous infection, so excluding previously convalescent individuals prevented this from affecting the conclusions.
Specifically, the researchers were identifying hospitalizations and deaths that were accompanied by a primary COVID-19 diagnosis, or a positive RT-PCR test within 30 days of death (normally beforehand).
Individuals were split into those who had been unvaccinated, partially vaccinated, and completely vaccinated (received both doses over 14 days prior to hospitalization or death).
Further factors considered included the number of previous positive tests, age, health region, sex, deprivation index, chronic conditions, and previous vaccines uptake.
Hospitalization
The total hospitalization and death rates per 1,000 person-years for unvaccinated, partially vaccinated and fully vaccinated individuals were estimated. Individuals vaccinated with alternate vaccine types contributed to unvaccinated person-time before vaccination.
Vaccine effectivity was estimated separately for those aged 65-79 and over 80 and calculated using a confounder-adjusted hazard ratio as estimated by time-dependent Cox regression. Around 1.4m people were examined, with multiple different vaccines, the most common of which were Comirnaty and Spikevax.
65-79
For the participants between 65-79, the results looked very promising. In cases where individuals had only received one dose of the vaccine, it showed 77% effectiveness, rising to 96% following the second dose.
Over 80s
In the over 80s, the findings were slightly less fortunate, with 56% effectivity from the first dose and 81% from the second. These findings coincide with and are supported by the results of multiple other studies showing decreased vaccine effectivity with age.
The study covered the original strain of COVID-19 found in Wuhan, as well as additional variants of concern such as the Alpha and Delta variants.
The authors highlight the high levels of protection that two doses of the vaccine provide, and these results strongly support a public health policy focused on mass vaccination programs.
The researchers theorize that the reduced effectivity seen in the elderly is likely due to age-related immunosenescence - the inability of immune cells to replicate would hinder the ability to quickly rebuild specific antibodies.
The study does have some weaknesses - primarily due to issues with the Portuguese national health database. Temporary/occasional users of this database and update issues could have artificially inflated the population studied.
Several exclusion criteria were created in order to prevent this, and final results show similarity to the official national statistics, indicating the success of these criteria. However, the researchers also firmly push for the consideration of additional vaccines doses and monitoring vaccines efficiency over time, given the evidence of reduced effectivity past 5-6 months if only two doses are administered.
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