> There are many diseases that have killed millions and been around for decades that still have no vaccine (eg HIV, malaria, dengue fever).
Dengue does have a vaccine, but like many vaccines, it isn't 100% effective. While it does reduce the chances of being infected, it also increases the likeyhood of a severe infection (similar to the risk increase from a previous dengue infection) and thus is only recommended for those who have already been been infected once.
Malaria also has a vaccine. It also is not 100% effective but it does reduce infection rates.
In fact, many vaccines are like this and confere some level of resistance rather than full immunity. This further illustrates your point that vaccines and immunity have a wider range of forms than commonly understood and we don't really know what form of vaccine is possible for Covid-19.
Edit: Another example is the typhoid vaccine I got last fall, it is only 50-80% effective.
>Dengue does have a vaccine, but like many vaccines, it isn't 100% effective. While it does reduce the chances of being infected, it also increases the likeyhood of a severe infection (similar to the risk increase from a previous dengue infection) and thus is only recommended for those who have already been been infected once.
Yeah, it's quite new. One of the issues with Dengue is that there are multiple types of the virus. If you get Dengue once, you're mostly protected against getting the same type again. But if you get one of the other types, you're at significant risk of severe complications (e.g. hemorrhagic fever). (ADDED: Hence, why you get the vaccine if you have had Dengue before.)
Dengue does have a vaccine, but like many vaccines, it isn't 100% effective. While it does reduce the chances of being infected, it also increases the likeyhood of a severe infection (similar to the risk increase from a previous dengue infection) and thus is only recommended for those who have already been been infected once.
Malaria also has a vaccine. It also is not 100% effective but it does reduce infection rates.
In fact, many vaccines are like this and confere some level of resistance rather than full immunity. This further illustrates your point that vaccines and immunity have a wider range of forms than commonly understood and we don't really know what form of vaccine is possible for Covid-19.
Edit: Another example is the typhoid vaccine I got last fall, it is only 50-80% effective.