
Provided the puppies took mum's first milk (the colostrum) which is where the natural immunity comes from (provided mum has been utd with her boosters prior to being mated!) this should hold them until around week 5 after which this natural immunity starts to drop off. There is, as far as I'm aware, no connection between being nursed beyond the first milk, and being given natural immunity. It's all down to the colostrum. After the immunity to be gained starts to drop away, obviously the puppy is increasingly at risk, but as they are all different (and the only real way to know what the puppies NEEDS is via titre testing the individual puppy - expensive!) it's felt best to wait until 8 weeks before externally vaccinating, by which time it should be reasonably certain the vaccination will 'take'. Done too soon with natural immunity still there, it won't and is pointless. The 2 week gap (sometimes 3 depending on the vet) is to repeat the vaccination in the hope the second set will take even if the first set didn't. Belt and braces. Although some vets just vaccinate against Parvo/Lepto with the first set and do the full set next time.
If a puppy hasn't received the colostrum, then it's usual that it will be given an interim external vaccination, against Parvo and Leptospirosis, at around 6 weeks.
No puppy can safely go off his property, or should, until the full set of vaccination has been given. We also restricted where our adults went off to be exercised as even healthy dogs, utd with boosters, can walk infection back into the home - as you can on your shoes in fact.
I hope this helps!! But as always, be guided by your vet.
ps We now have the full sets of puppy vaccination given, and booster 12 months on. After that mine are now on 3-yearly boosters, apart from the one for Lepto. which dips and is felt needed to be boostered annually. Rabies annually in those countries living with this disease and according to local laws - the reality suggests this too is only really needed 3-yearly. And then there's what Boarding Kennels require.