Fully cooked, but requires a minimum internal temperature?

There are arguments of the universal validity of such rules, but there has long been an argument that reheated meat that was pre-cooked should always be brought to full safe temperature.

The general idea is that cooked or not there is bacteria present. If heated to only the 100-120F range, you have reached the temperature of highest active growth of many pathogens, without killing them. Some will argue that this is safe, as long as you eat the item immediately, not let it sit and give time for that bacterial growth to bloom. I would even put credence to this for items which I knew the preparation and handling history, but for prefab type foods, like ground meats and especially poultry, I personally would be reluctant to take risks, I would bring it to temp.

There also has always been disclaimers that the reheating to temp rule only applies to if you reheat at all, for instance, medium rare roast beef is often eaten cold as leftover, but the rule stated if you reheat, the reheat to what would have originally be considered a safe temperature, not part way. With something like a prepped burger, again, I personally would not eat it cold, and just thawing it might put you into an unknown zone. I would heat it.

A few things to add:

1. Cooking does not necessarily kill all bacteria or other dangerous microorganisms. Some bacteria form spore forms which can survive fairly high temperatures. (Botulism bacteria can even survive boiling temperatures.) Thus, even assuming food was cooked properly, it often still harbors bacteria.

2. Spore forms can reactivate when cooled. If the food is cooled over too long of a period, the bacteria growth can make the food unsafe to eat. (In fact, a lot of food poisoning is due to mishandling of leftovers, since fresh foods often have competition among various microorganisms, but leftovers sometimes only have bad bacteria in spore form which can then grow rapidly.)

3. Temperature mishandling during storage can also be a problem. Most previously cooked food is supposed to be continuously refrigerated or frozen, but food might sometimes sit out during transport or something. The same growth mentioned in the previous point can occur.

4. Processed foods can be contaminated during handling after cooking. This is one of the most common reasons for food recalls of "previously cooked" food: if the equipment for packaging is contaminated or if the food comes in contact with other non-cooked food during packaging, microorganisms can be transferred. (And once again, cooked food is relatively sterile, meaning that it's sometimes easier for bad microorganisms to grow quickly in it without competition against other microorganisms.)

5. If a "previously cooked" item is contaminated, reheating may not only destroy bacteria themselves, but sufficient temperatures can sometimes denature toxic byproducts of bacterial growth. This is sometimes the rationale for reheating food to higher temperatures: a lower temperature may be sufficient to kill bacteria, but if a lot of bacterial growth did occur in a contaminated item, the lower temperature may be insufficient to destroy toxins left behind.

Lastly, food safety protocols are often created by governments with a specific consumption pattern in mind. Regulations governing food that is meant to be consumed cold after cooking are sometimes different from regulations for food that is meant to be reheated. Additional processing steps are often taken to ensure cold food is safe (e.g., faster cooling, stricter temperature control, occasionally the use of preservatives, recipe modifications, or additives that serve to limit bacterial growth).

Cooking poultry so it is safe to eat is important – high temperature kills campylobacter and Salmonella, which are the most present in poultry that we worry about. Poultry should be tested with a temperature probe to 165 F (74 C). This nonsense about ‘cook thoroughly until it is steaming hot throughout, the juices run clear and there are no pink bits inside’ is not accurate or safe. Be safe and use a temperature probe. There are a lot of reasons why cooked poultry might be pink… read: http://www.epicurious.com/expert-advice/safe-eat-bloody-chicken-article