As I thought I understood

, the body becomes sensitised to the adenovirus such that even if it did confer some spike protein reaction, it would cause increasingly strong immune responses to the adenovirus - thus increasing "side" effects.
Maybe both aspects are significant in terms of delivering an effective vaccine?
The mRNA platform vaccine is not affected by this as well as the Sputnik V vaccine from Russia, which is Adenovirus based with 2 types; Adeno Type 5 and Adeno Type 26 and they are administered separately and thus won't allow the body to be sensitized to the vector as easily, but still able to trick the body to mount a response to the engineered antigen in the differing vector instead. The UAE is a country which is using the SinoPharm (inactivated virus), Pfizer Biontech (mRNA) and Sputnik V (Adenovirus vector) to allow them plenty of options for the booster shot if necessary.
Regardless, there has been UK discussion about switching between Pfizer and AstraZeneca for many, or (if available) going for the upcoming Novavax (maybe Janssen, Moderna or something else).
Can't help wondering what really happens if two different vaccines are used? Do they end up re-provoking the response achieved by the first vaccine, or a de novo response to the new one, or something more complex?
So far, studies had shown that using 2 different platforms (Adenovirus + mRNA) will actually mount a higher immune response to the virus rather than using the same platform, seemed safe enough with manageable side-effects and produce more of the potent neutralizing antibodies against the variants. Canada is one of the countries that actually experimented with this, but later abandoned in favor of mixing Pfizer and Moderna when necessary because of the bad press Astra got and monetary compensations being paid out to people who got permanent side effect from Astra.
Another issue during the mixing experiment is that of branding preference and vaccine passport. On the street, the Pfizer Biontech vaccine is known as the Bourgeois of vaccine (upper class), Moderna being middle class and the Adenovirus being lower class because of the bad rap it got with the AstraZeneca and its blood clot side effects with a certain population. In Canada, it caused people to cancel appointments when they found out they are getting the Moderna or a mixed mRNA brand (eventhough it's based on the mRNA platform like Pfizer), and then reschedule intentionally so they can get the Pfizer, because they want their future vaccine passport in a form of an ArriveCan app to show Pfizer given for both shots or Moderna given for both shots. In some other countries, this causes confusion in vaccine passport eligibility where 2 of the same platform vaccines (namely Pfizer or Moderna) are recognized as fully vaccinated, but 2 differing platforms are not. This has caused unnecessary wasting of close to expiry Moderna and AstraZeneca vaccines which can save lives. This is not limited to only Canada, but around the world. Wasted vaccines do not save lives.
However, the plan I think with many nations is that by the time a booster shot is required is that the 2nd generation vaccine, which is the more traditional "protein-based with an adjuvant" will become approved for use. The top contender for this protein based vaccine is none other than Novavax. You probably heard this sometimes in the news with the anti-vax people saying that, they will only get vaxed with Novavax or with Sanofi-GSK which is another protein based vaccine. In Canada, we have Medicago-GSK vaccine from Quebec's firm Medicago which is a "plant based" adjuvant vaccine based on its "Proficia platform". In fact, I actually wanted to wait for the Medicago vaccine myself as the preferable platform, but took the mRNA vaccine to recover from LONG Covid. Protein based vaccine with an adjuvant is safe, trusted and easier to store in fridge temperature of 2-8 degrees Celsius as opposed to the Pfizer Biontech -70 degrees Celsius. So the Novavax' 2-8 Celsius storage for a minimum of 6 months is a plus. It can be stored in a doctor's fridge and can be administered broadly to the population. And besides, Protein based vaccines should promote a longer lasting immune response that can last at least 3 years and should be ok for mixing with other platforms as boosters, but like anything else more studies will need to be conducted to evaluate its safety and efficacy.
There's a saying that "good things will come to those who wait". I suspect that moving forward, many of us fully vaxed will probably get the Novavax or the Sanofi-GSK as boosters if necessary and those would become the dominant vaccines used to vax the global population and I suspect that Medicago-GSK would not be far behind with Canada using it to boost its domestic population and sharing the rest with Covax/Gavi on a humanitarian basis. Costs should become more reasonable with protein based vaccines and you don't need those specialized cold freezers which makes transportation and distribution of these vaccines to the world through Covax/Gavi much easier logistics wise.
The thing is that, we will not be free of COVID until all of us are free of COVID and that means vaccinations. Right now, the trend unfortunately is very predictable with first world nations getting vaxed, but the poorer countries like Africa not getting vaxed plenty enough. This is the same trend as the Rotavirus vaccine such as RotaTeg which was used in 2006 in the US and later RotaTrix in 2008, given in 3 doses to address the symptoms of vomiting and diarrhea after exposure to the Rotavirus. Despite the availability of these vaccines, more than 11 years later roughly 60% of the population of children still have yet to receive this vaccine. That's the inequality of vaccine distribution you will often see between rich and poor nations, but through Covax and Gavi (The Vaccine Alliance), I hope this inequality will be addressed well with the COVID vaccine and should be. Otherwise, you'll have instances when the border opens fully up and we have cross border travel and migrations that pockets of COVID infection will continue to happen if the global population are not fully and equally vaxed to achieve herd immunity, because so far 4 billion doses of COVID vaccines had been given to higher and middle income countries, eventhough they account for less than half the world population.